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University  of  California. 


T  I  FT    OK 


THE  NEBULA  IN  ORION 


,:,!)  L863 


WASHINGTON  ASTRONOMICAL  OBSERVATIONS  FOR  1878— APPENDIX  I. 


MONOGRAPH 


OF   THE 


[R.  A.  5h  28m  248.o;  N.  P.  D.  95°  29'  io".9;   1 860.0.] 


BY 


EDWARD    S.    HOLDEN, 

PROFESSOR     OF     MATHEMATICS,     U.     S.     NAVY. 


WASHINGTON: 

GOVERNMENT    PRINTING   OFFICE. 
1882. 


TABLE    OF    CONTENTS. 


Pag 

INTRODUCTION 

Object  of  the  research 

System  of  nomenclature  adopted 

Catalogue  of  stars 

Index-chart  of  the  central  parts 

List  of  the  drawings  referred  to ' 

List  of  the  memoirs,  etc.,  on  the  nebula  of  Orion 

List  of  telescopes  employed  by  the  various  observers 

PART  I.  History  of  the  various  researches  on  the  nebula  of  Orion,  in  chronological  order 

CYSAT'S  discovery  of  the  nebula  (1618) 

HUYGHENS'  observations  (1656).     See  Fig.  i 

HOOKE'S  observations  (1666) 

HUYGHENS' observations  (1694).     See  Fig.  2 /    .     . 

MAIRAN'S  observations  (1731).     See  Fig.  3 

PICARD'S  observations  (1673).     See  Fig.  4 

LONG'S  observations  (1742).     See  Fig.  5 : 

LEGENTIL'S  observations  (1758).     See  Fig.  6 : 

Washington  drawing  of  1877,  February  3,  for  comparison  with  early  figures.     See  Fig.  7 : 

Comparison  of  the  uncorrected  outlines  of  HUYGHENS',  PICARD'S,  and  LEGENTH/S  drawings.     See  Fig.  8       .         : 

Comparison  of  the  corrected  outlines  of  these  three  drawings.     See  Fig.  9 : 

MESSIER'S  observations  (1771).     See  Fig,  10 ' 

W.  HERSCHEL'S  observations  (1774-1811).     See  Fig.  n '• 

Discussion  of  the  variations  in  form,  etc.,  according  to  HERSCHEL ; 

Drawing  by  Mr.  J.  E.  KEELER  (1878)  for  comparison  with  HERSCHEL.     See  Fig.  12 ; 

Situation  of  the  trapezium  with  regard  to  the  adjacent  nebulosity,  according  to  all  the  early  authorities    .      .         ; 

LEFEBVRE'S  observations  (1779).     See  Fig.  13 : 

SCHROETER'S  observations  (1794-1798),     See  Figs.  14,  15,  16,  17 ; 

Discussion  of  SCHROETER'S  observations ' 

BOOK'S  observations  (circa  1800).     See  Fig.  18 < 

FLAUGERGUES' observations  (1802) * 

J.  HERSCHEL'S  observations  (1824).    See  Fig.  19 

POND'S  observations  (1826) ! 

LAMONT'S  observations  (1837).     See  Fig.  20 ! 

J.  HERSCHEL'S  observations  (1837).     See  Fig.  21 ! 

DE  Vico's  and  RONDONI'S  observations  (183^-1 841).     See  Fig.  22 ! 

KAISER'S  observations  (1844).     See  Fig.  23 

COOPER'S  observations  (1847?) ! 

LASSELL'S  obserTations  (1847).     See  Fig.  24 

W.  C.  BOND'S  observations  (1848).     See  Fig.  25 

LIAPONOFF'S  and  STRUVE'S  observations  (1847-1851).     See  Fig,  26 

LASSELL'S  observations  (1854).     See  Fig.  27 ' 

SCHMIDT'S  observations  (1860-1875).     See  Fig.  28 

SECCHI'S  observations  (1862).     See  Fig.  29 

TEMPEL'S  observations  (1862).     See  Fig.  30 

LASSELL'S  observations  (1862).     See  Fig.  31 

3 


4  TABLE  OF  CONTENTS. 

PART  I— Continued.  Page. 

STRUVE'S  observations  (1863) 77 

WEBB'S  observations  (1863-1876) 78 

BIRD'S  observations  (1866) 81 

G.  P.  BOND'S  observations  (1859-1865).     See  Frontispiece  and  Fig.  32 8r 

Lord  ROSSE'S  observations  (1867).     See  Fig.  33 85 

SECCHI'S  observations  (1868).     See  Fig.  34 91 

D'ARREST'S  observations  (1872).     See  Fig,  35 98 

WINLOCK  and  TROUVELOT'S  observations  (1874).     See  Fig.  36 104 

TROUVELOT'S  drawing  (1875).     See  Fig.  37 104 

DOBERCK'S  observations  (1877,  1878) 104 

TROUVELOT'S  observations  (1876?) 104 

LANGLEY'S  observations  (1879).     See  Fig.  38     .     .^ 105 

PART  II.  Washington  observations  of  the  nebula  of  Orion,  in  chronological  order  (from  1874,  January  u,  to  1880, 

January  29) 108 

Synopsis  of  the  preceding  detailed  observations 167 

Collection  of  the  separate  results  of  observations  of  stars 179 

Stars  within  the  trapezium 179 

STRUVE'S  variable  stars 180 

Observations  of  variable  stats 181 

Synopsis  of  the  more  important  measures  made  at  Washington 184 

Comparison  of  the  positions  of  nebulous  masses,  as  determined  at  Kasan  and  Washington 190 

Reduction  of  photometric  observations  made  at  Washington igi 

I.  Results  from  eye  observations  of  the  order  of  brightness  of  the  principal  masses 192 

II.  Observations  with  HASTINGS'  photometer 191; 

Description  of  the  instrument.     See  Fig.  39 ig6 

Results  from  these  observations 197 

PART  III.  Summary  of  all  the  observations  (1656-1880),  which  have  previously  been  given  in  detail 200 

Spectrum  of  the  nebula,  according  to  HUGGINS,  LESUEUR,  VOGEL,  SECCHI,  and  D'ARREST 221 

PART  IV.  Conclusions 224 

ADDENDUM.  Dr.  DRAPER'S  photographs  ot  the  nebula  in  Orion  (i88o-'8i-'82).     See  Fig.  40 226 

NOTE.— The  important   results  obtained  by  Dr.  HUGGINS   from  his  photographs  01  the  spectrum  oi   this   nebula 
were  known  too  late  for  insertion  in  the  text. 


MONOGRAPH 


CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


The  main  object  of  this  memoir  is  to  leave  such  measures  and  descriptions  of  the 
brightest  parts  of  the  nebula  of  Orion  as  shall  enable  another  person  observing  in 
after  years  with  the  same  telescope,  under  like  conditions,  to  say  with  certainty  whether 
or  no  changes  have  occurred  in  these  parts  of  this  nebula.  The  brightest  parts  are 
chosen  so  as  to  avoid  as  far  as  possible  any  uncertainty  in  the  conclusion  then  to  be 
reached,  and  also  because  there  is  little  to  be  added  to  the  complete  observations  of 
Lord  ROSSE  and  of  Gr.  P.  BOND  on  the  fainter  portions.  A  second  and  an  important 
object  is  to  make  a  thorough  discussion  of  the  vast  mass  of  material  now  on  hand. 

The  brilliant  success  attained  by  Dr.  HENRY  DRAPER  in  his  photographs  of  this 
nebula  leads  to  the  hope  that  photographs  may  in  the  near  future  largely  take  the 
place  of  eye-drawings  for  such  objects  as  comets  and  nebula?;  the  present  is  an  appro- 
priate time  for  a  resume  of  all  observations  made  by  the  old  methods. 

A  complete  research  on  this  nebula  might  treat  of  the  following  questions: 
I.  Its  distance  from  the  earth. 
II.  Its  connection  with  the  stars  contained  in  it. 

III.  Its  physical  constitution. 

IV.  Its  proper  motion. 

V.  The  variations  in  the  shape  of  its  parts. 
VI.  The  variations  in  the  brightness  of  its  parts. 
VII.  Its  possible  rotation  as  one  mass. 
VIII.  The  possible  rotation  of  one  or  more  of  its  parts. 

IX.  The  proper  motion  of  one  or  more  of  its  parts. 

The  present  memoir  affords  evidence  relating  to  II,  IV,  V,  VI,  VII,  VIII,  and  IX. 
II  has  also  been  treated  in  the  Washington  Observations  for   1877,  Appendix  I, 
"On  the  Multiple  Star  2.  748,"  where  I  have  discussed  a  most  complete  set  of 
measures  of  the  six  stars  of  the  trapezium  made  by  Professor  HALL, 

The  object  to  be  attained  could  not  have  been  reached  by  adding  another  drawing 
to  the  many  excellent  ones  we  now  have,  and  my  original  plan  of  making  micrometric 
and  photometric  measures  of  the  various  masses  has  been  carried  out  without  much 
change.  All  the  observations  at  Washington  have  been  made  with  the  26-inch  Clark 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION, 


refractor,  with  magnifying-  powers  from  130  to  600  diameters.  A  full  description 
(with  plates)  of  this  telescope  is  given  in  Washington  Astronomical  Observations, 
1874,  Appendix  I.  A  view  of  it  is  given  in  the  accompanying  wood-cut  from  ANDRE 
and  RAYET'S  Astronomic  Pratique,  kindly  furnished  by  M.  GTAUTHIER-VILLARS. 

Probably  no  object  outside  of  the  solar  system  has  received  more  attention  from 

the  best  observers  than  the  nebula  of 
Orion.  Before  discussing  the  obser- 
vations of  so  many  astronomers,  each 
of  whom  has  applied  his  own  pecu- 
liar notation  to  the  various  parts  of 
this  nebula,  it  is  necessary  to  fix  upon 
one  system  of  nomenclature  which 
shall  be  used  uniformly  throughout 
the  work.  For  the  stars  no  doubt 
can  arise  as  to  the  proper  system  to 
be  adopted,  as  the  Catalogue  of  Stars 
in  the  Nebula  of  Orion,  published  by 
Gr.  P.  BOND  in  vol.  v  of  the  Annals  of 
the  Harvard  College  Observatory,  is  bv 
far  the  most  full  that  we  possess,  and 
is  likely  to  remain  so  for  many  years. 
I  have  uniformly  adopted  the 
nomenclature  of  all  stars  in  the  neb 
ula  from  that  great  work,  and  while, 
in  quoting  from  other  authorities,  I 
have  given  in  most  cases  the  letter  or  number  of  the  star  from  the  original  source,  I 
have  added  the  synonym  from  BOND  in  square  brackets,  thus:  [G.  P.  B.,  No  685,  etc.] 
or  simply  [685].  I  give,  immediately  following,  a  catalogue  of  all  the  stars  referred 
to  in  the  subsequent  pages  reduced  to  1877.0  from  BOND'S  elements.  This  catalogue 
forms  the  basis  of  the  present  work. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  7 

Catalogue  of  Stars  in  the  Central  Part  of  the  Nebula  of  Orion  for  1877.0. 


BOND. 

Mag. 

HERSCHEL 
and 
STROVE. 

BOND'S  let- 
ter in  his 
Zones. 

HERSCHEL. 

W.  C.  BOND. 

LASSELL. 

LlAPONOFK. 

1877.0. 

Ad 
1877.0. 

479 

IO.O 

35 

F' 

0 

3 

56 

y 

-  400.8 

+  2  72"  5 

523 

10.  I 

45 

P" 

r 

5 

40 

/ 

-  243.1 

—  116.2 

558 

10.7 

50 

Q" 

9 

39 

V 

-  159-7 

--  119.0 

*5&7 

13.9 

51 

IL 

10 

. 

. 

-  103.4 

-       8.7 

570 

9-4 

53 

R" 

a 

13 

33 

n 

—   96.0 

-  273.6 

573 

13-9 

54 

r*' 

. 

12 

/35 

«i 

-    87.4 

-   1/9.4 

*575 

11.9 

57 

r" 

. 

II 

45 

• 

85.4 

—     22.7 

581 

14.2 

rt</54 

r*1 

. 

. 

. 

-    77.1 

-   159-5 

»  *5§9 

12.7 

57* 

r\ 

15* 

. 

. 

-     57-8 

—      21.0 

*595 

13.9 

v 

. 

15 

43,* 

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15-6 

. 

. 

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—      32.0 

602 

14-3 

v' 

. 

. 

-     33-6 

—     68.1 

*6o8 

14-3 

v' 

. 

/ 

-     24.3 

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*6l2 

13-5 

f 

16 

-     17.0 

4-     24.0 

*6i7 

64 

r 

y 

— 

bbi 

-     10.7 

4-     12.9 

*6i8 

I3-I 

7T 

. 

19 

h 

—       II.  0 

4-     24.0 

*6ig 

65 

K' 

7 

17 

b 

—      IO.O 

4-       8.7 

*62I 

15.6 

ad  11 

. 

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*622 

12.7 

II 

P 

. 

18 

. 

-       8.1 

28.4 

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67 

U";  L' 

6 

21 

d 

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4-     16.1 

625 

15-6 

ad  II 

. 

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d 

• 

—       5.0 

—     29.0 

628 

. 

69 

M' 

'a 

22 

a 

o.o 

0.0 

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14-3 

r 

• 

• 

4-         2.0 

43-0 

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.      . 

71 

N' 

a' 

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+     3.5 

—         2.1 

635 

10.5 

70 

O' 

23 

2 

i 

4-    7.9 

+    97.7 

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24' 

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4-       7-8 

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73 

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25 

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14.8 

III 

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4-     ii.  5 

4-   no.  6 

*&42 

15-6 

v' 

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4-     12.0 

4-     47-0 

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12.  I 

75 

*;>»' 

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26 

9i' 

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4-       22.0 

+     37-4 

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14-3 

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. 

• 

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4-     23.6 

-       9-3 

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I3-I 

0^75 

i/) 

. 

27 

+     28.8 

+     47-0 

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13-9 

76 

c 

/ 

32 

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f" 

4-     30.0 

4-   170.8 

*654 

12.3 

78 

u 

31 

4-     32.6 

4-       9.2 

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I3-I 
II  .  7 

80 

84 

s, 
6 

w 

33 
37 

4 

& 

+     39-4 
+     55-1 

4-   164.4 
4-   M6.3 

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•*••*•/ 

13.9 

81 

Xs" 

30 

. 

+     58.7 

—   196.6 

669 
*67i 

9.8 

8? 
88 

Q' 

v 

39 
41 

10 

18 

k 

4-     62.9 
4-     69.0 

4-     99-2 
—     25.2 

*675 

15-2 
13.  i 

ad  88 

Y 

• 

43 

a 
k 

K 

+     73-7 
+     77-9 

-     94-2 
-     28.4 

*677 
*68i 
685 
*6S6 

14.8 
14.8 

8.3 
15.6 

89 
93 

Xa" 

n 
Y" 

z 
e 

34 

45 
44 

26 

e 

+     77-6 
+     90.1 
+     96-9 
4-     99-0 

—    202.2 

4-   172.4 
-     95.8 
—     40.0 

•X-AQO 

4-   105.0 

—     19.0 

*688 

707 
708 

15.6 

II.  2 
9.6 

.103 

101 

A"  A" 
B"  B" 

C 

49 

50 

27 
23 

a 
f 

4-   150.2 
4-   150.6 

-  254.5  ; 
—  99.5 

12.  ^ 

IOO 

b"  i>i" 

G 

51 

H 

4-   152.1 

-  137-4 

724 
741 
746 

784 

88g 

j 
10.5 
IO.O 

10.8 
10.8 
ii.  3 

104 
no 
in 
123 
142 

C"  C" 
E"  E" 

"H"  H" 

K"  K" 

E 
K 
(*) 

55 
61 
64 
78 
94 

25 
19 
29 
24 
17 

h 

n 

+   182.3 
4-  225.1 
4-  231.3 
+   387.2 
4-   800.4 

-  177.0 

-   in.  7 
-   585-0 
-  287.4 
—  256.0 

-.*-      C'T'Tl  I*  X*  V 

The  magnitudes  are  from  G.  P.  BOND'S  estimates. 


8  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

It  is  equally  necessary  that  a  rather  minute  system  of  nomenclature  should  be 
adopted  to  distinguish  the  various  bright  masses,  dark  channels,  spirals,  etc.,  of  the 
central  portion,  and  the  Index-Map,  herewith,  gives  the  nomenclature  uniformly  used 
throughout  the  present  work,  not  only  in  referring  to  my  own  observations  of  1874, 
'75>  '76>  '77?  '7^  '79?  an(i  l88o>  but  m  tne  discussion  of  the  work  of  others. 

It  is  necessary  to  say  a  few  words  in  regard  to  the  nomenclature  there  adopted, 
as  it  is  not  all  that  could  be  wished.  During  a  visit  of  M.  TROUVELOT,  formerly  of  the 
Harvard  College  Observatory,  to  Washington,  he  made,  in  connection  with  myself,  the 
drawing  of  the  central  part  of  nebula  Orionis  which  is  reproduced  in  Appendix  I, 
Washington  Astronomical  Observations  for  1874.  This  was  only  a  preliminary  sketch, 
but  it  gave  an  idea  of  what  could  be  seen  with  the  26-inch  refractor.  On  his  return 
to  Cambridge  I  requested  him  to  prepare  a  lithographed  skeleton  map  of  the  central 
portions  of  the  nebula,  on  which  I  proposed  to  insert  letters,  figures,  etc.,  to  designate 
the  various  bright  masses,  dark  channels,  etc.  I  intended  to  choose  these  symbols  so 
as  to  preserve,  not  only  the  nomenclature  proposed  by  Sir  JOHN  HERSCHEL  in 
Memoirs  of  the  Royal  Astronomical  Society,  vol.  ii,  which  has  been  adopted  and  added 
to  by  subsequent  investigators,  but  also  the  nomenclature  of  the  bright  masses 
(<*,  /?,  7,  etc.)  given  in  Lord  ROSSE'S  memoir  of  1868  (Phil.  Trans.,  1868,  p.  57),  and 
in  LIAPONOFF'S  Memoir  published  by  STRUVE  in  Memoires  de  V Academic  Imperiale  des 
Sciences  de  "St.  Petersbourg,  vol.  v,  7th  series,  1862.  The  brilliant  labors  of  Lord  EOSSE, 
LIAPONOFF,  and  STRUVE  in  this  field  demanded  that  the  nomenclature  adopted  by  them 
should  not  be  lightly  changed. 

On  the  return  of  the  lithographed  charts  (which  were  executed  by  M.  TROUVELOT 
at  his  own  expense),  however,  I  found  on  them  a  system  of  letters  and  numbers 
excellent  in  itself,  -but  varying  from  the  nomenclatures  of  ROSSE,  LIAPONOFF,  and 
STRUVE.  These  charts  were  put  into  immediate  use  in  my  own  work,  and  copies  of 
them  were  sent  to  Dr.  DOBERCK,  Mr.  PRITCHETT,  Lord  ROSSE,  M.  OTTO  v.  STRUVE,  Dr. 
SCHMIDT,  M.  TEMPEL,  M.  TISSERAND,  Dr.  VOQEL,  Dr.  WINNECKE,  and  others ;  and  some 
of  these  were  at  once  used  in  comparisons  with  the  nebula  by  these  astronomers.  They 
were  constantly  used  in  my  own  work,  and  thus  almost  unavoidably  a  nomenclature 
was  adopted  which  did  not  fulfill  all  the  prerequisite  conditions.  Added  to  this  was  the 
fact  that  the  nomenclatures  adopted  by  former  astronomers  were  not  then  as  familiar  to 
me  as  now,  as  it  was  my  constant  endeavor  while  the  actual  work  was  in  progress  to 
keep  my  mind  as  free  from  bias  as  possible,  and  to  avoid  too  great  familiarity  with 
previous  work.  That  this  process,  while  advantageous  from  a  purely  scientific  point 
of  view,  has  also  its  disadvantages,  the  preceding  remarks  will  show. 

I  have  seriously  considered  the  question  of  changing  my  nomenclature  through- 
out on  these  accounts,  but  the  fear  of  introducing  error,  and  the  chance  that  these 
charts  may  also  be  used  by  the  astronomers  to  whom  they  were  sent  has  deterred  me, 
and  the  index-chart  herewith  remains  substantially  as  it  has  been  during  the  series 
of  observations.  I  have,  however,  added  the  nomenclature  of  LTAPONOFF  in  many 
cases,  distinguishing  his  letters  by  inclosing  them  in  a  right  angle.  Lord  ROSSE'S  Greek 
letters  are  underscored  in  the  index-map  to  distinguish  them  from  M.  TROUVELOT'S. 


U 


t  III. 


with  th 


i  LON> 

;  w. } 


1824 
1826 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


DESCRIPTION    OP    THE    INDEX-CHART. 

The  stars  are  laid  down  from  BOND'S  Catalogue,  and  the  numbers  are  throughout 
those  of  BOND.  The  stars  inclosed  in  circles  are  those  suspected  by  STRUVE  to  be 
variable,  and  were  so  distinguished  in  order  to  attract  the  eye,  except  the  star  h. 
Those  stars  in  triangles  were  stars  laid  down  by  LASSELL,  which  my  own  early  obser- 
vations had  not  verified.  The  letters,  numbers,  etc.,  were  laid  down  by  M.  TROU- 
VELOT,  and  are  very  convenient  for  the  purpose  for  which  they  were  intended.  I 
have  kept  the  nomenclature  of  HERSCHEL,  Sinus  magnus,  regio  Huygheniana,  etc.,  as 
it  is  now  classic,  and  I  have  added1  but  one  such  term,  and  this  was  done  almost 
by  inadvertence.  The  sharp  following  point  of  6  I  have  called  "  Spitze." 

The  dimensions  of  the  various  masses  in  the  Index-Chart  are  not  strictly  accu- 
rate, although  nearly  so.  For  accurate  dimensions  recourse  must  be  had  to  my 
measures  in  Part  III.  The  index-chart  is  simply  intended  as  a  key  to  the  system  of 
nomenclature,  and  to  make  verbal  descriptions  intelligible. 

The  principal  drawings  referred  to,  with  their  dates,  are  given  in  the  following 
list,  in  which  the  order  is  the  same  as  that  adopted  in  the  subsequent  discussion : 


Observer. 

Date. 

In  what  published,  etc. 

Figure 
in  this  work. 

HUYGHENS            .       . 

1656 

I 

PlCARD    .... 

HUYGHENS 

1673 
1694 

Traite"  de  1'Aurore  Bore"ale,  p.  248,  and  Hist,  de  1'Acad.  Roy.  des  Sci- 
ences, 1759,  p.  435. 

4 

2 

MAIRAN  .... 

1731 
1742 

Traite"  de  1'Aurore  Bore"ale,  p.  249,  and  LALANDE'S  Astronomic,  p.  272 

3 
c 

1758 

6 

1771 

10 

W.  HERSCHEL  . 

1774 
and  later, 

I77Q 

P.  T.,  1811,  p.  320,  and  MSS.  in  possession  of  the  Royal  Society  of 
London. 

ii 
19 

I7OA 

14 

I  "707     '08 

15,  16,  17 

BODE      .... 

1800? 

1802 

Anleit.  z.  Kenntniss  d.  Gest.  Himmels,  p.  166  and  Plate  i    .      .     .     . 

18 

1824 

19 

POND      .... 
J.  HERSCHEL     . 
LAMONT  .... 

1826 
1837 
1837 

TQ/1  "3 

Mem.  R.  A.  S.,  ii,  p.  93  
Obs.  Cape  of  Good  Hope,  pp.  25  et  seq  
Ueber  d.  Nebelflecken,  p.  23,  and  MS.  drawing  kindly  communicated 
by  Dr.  DOBERCK. 

21 

20 

Io3r 

22 

1039 

rQ  AT 



23 

1844 

24 

LASSELL  .... 
W.  C.  BOND      .     . 

1847 
1848 

Mem.  Amer.  AcacL,  vol.  iii,  p.  87,  and  Annals  H.  Coll.  Obs'y,  vol.  5    . 

25 
27 

SCHMIDT      .     .     . 
SCHMIDT      .     .     . 

!°54 
i86o-'75j 
1861     ] 

Two  MS.  unpublished  drawings,  courteously  communicated  by  Dr. 
SCHMIDT. 

\          " 

APP.  V- 


10 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

The  principal  drawings  referred  to,  with  their  dates,  etc. — Continued. 


Observer. 

Date. 

In  what  published,  etc. 

Figure 
in  this  work. 

B.  B.  STONEY    .     . 
LIAPONOFF  .     .     . 

1851 
1847    > 

Unpublished  drawing.     A  photograph  of  this  has  been  kindly  given 
me  by  Lord  ROSSE. 

26 

O.  STRUVE  .     .     . 

1861    ) 

T8A/1 

2Q 

SECCHI    .... 

« 
rQAo  ? 

3° 

LASSELL  .... 

1862 

Unpublished  drawing,  on  a  large  scale,  made  by  Miss  CAROLINE  LAS- 
SELL,  at  Valetta,  of  which  a  full-sized  fac  simile  has  been  most  kindly 
communicated  by  the  artist. 

3i 
Frontispiece 

TQA*7 

and  Fig.  32. 
M 

1868 

Firenze  Ital.  Soc.  Mem.,  vol.  i,  $d  ser.,  pt.  2      

14 

rRT? 

•»e 

WINLOCK     .     .     . 
TROUVELOT  .     .     . 

|      1874     | 

Ast.  engravings  from  Harvard  College  Observatory,  and  Annals  Har- 
vard College  Observatory,  vol.  viii. 

|                36 
17 

LANGLEY      .     .     . 

1075 
1879 

l88o 

MS.  observations,  kindly  communicated  by  Professor  LANGLEY,  with 
the  permission  of  the  Superintendent  of  the  U.  S.  Coast  Survey. 
From  photographs  taken  in  1880  

38 
4° 

I  add  here  a  list  of  writings  on  the  nebula  of  Orion,  reprinted  with  additions 
from  my  Index- Catalogue  of  Books  and  Memoirs  relating  to  Nebulce  Clusters,  etc.  Wash- 
ington, 1877.  (Smithsonian  Miscellaneous  Collections,  No.  311.) 

LIST  OF  THE  MORE  IMPORTANT  BOOKS  AND  MEMOIRS  RELATING  TO  THE  NEBULA 

OF  ORION. 


ARAGO  :  C.  R.,  xiii,  p.  450. 
:  C.  R.,  xxvi,  p.  50. 


[Remarks  on  RONDONI'S  drawing.] 
[BOND'S  drawing.] 

BARNEBY  :  Mon.  Not.  R.  A.  S.,  vol.  xxxiv,  p.  248.     [Variability  of  6th  star  in  trapezium.] 
BESSEL:  B.  J.,  1808,  p.  122.     [CYSAT  knew  of  the  nebula  of  Orion.] 
BISHOP:  [HIND]  :  BISHOP'S  Astron.  Obs.,  i839-'5i,  p.  12  ;   1852.     [Small  star  near  6  Orionis;  mag.  13,^  = 

1260.9(3);  J=2".8(i). 
BODE:  Anleitung  z.  Kenntniss  des  Gestirnten  Himmels,  p.  166,  Plate  i,  p.  556.     [Two  drawings.] 

:  Himmelskarten,  Tafel  30.     [Drawing.] 

BOND,  G.  P.:  Annals  Harvard  College  Observatory,  vol.  v,  1867.     [With  two  steel  engravings  and   two 

charts.]     4°. 

:  Mon.  Not.  R.  A.  S.,  xxi,  p.  203.     [Spiral  structure.] 

:  Mon.  Not.  R.  A.  S.,  xxiv,  p.  177. 

BOND,  W.  C. :  Description  of  the  nebula  about  0  Orionis,     Mem.  Am.  Ac.  Arts  and  Sciences,  vol.  iii  (1848), 

p.  87.     [With  steel  engraving.] 

:  Proc.  Am.  Ac.  Arts  and  Sciences,  i,  p.  325.     [Observations.] 

:  Same  volume,  p.  342.     [Resolvability  of  nebula  of  Orion.]     See  also  Am.  Jour.  Sci.,  2d  series,  iv, 

p.  427. 

CARPENTER  and  STONE  :  Mon.  Not.  R.  A.  S.,  xxiv,  p.  92.     [On  G.  P.  BOND'S  drawing.] 
CASSINI,  J.  D. :  De  Cometa  Anni  i652-'53.     [Discovery  of  the  4th  star  in  Orion's  trapezium,  etc.] 
:  Decouverte  de  la  lumiere  celeste  qui  parait  dans  le  Zodiaque.     [Suspects  nebula  of  Orion  to  be  a 

star  cluster.]     See  DELAMBRE.     Hist,  de  1'Astr.  Mod.,  vol.  ii,  pp.  700,  709,  744. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION,  1 1 

CYSAT:  CYSAT,  der  erste  Entdecker  des  Orions-Nebel.     (1619.)     [R.  WOLF,  1853.] 

:  Mathemata  astronomica  de  loco  cometae  qui  sub  finem  anni  1618,  etc. 

D'ABBADIE  :  Mon.  Not.  R.  A.  S.,  xvii,  p.  245.     [PORRO'S  new  star  in  trapezium.] 

D'ARREST  r^Undersogelse  over  de  nebulose  Stjerner,  etc.     1872.     40.     [With  drawing  of  the  nebula  and 

detailed  memoir.] 

:  A.  N.,  Ivii,  col.  341. 

— :  A.  N.,  hex,  col.  337.     [Notice  of  LEFEBVRE'S  drawing.] 

:  Om  den  store  Orionstage.  Kjobenhavn.  Dansk.  Vid.  Selsk.  Oversigt.  (1867),  pp.  236-241.     [This 

paper  relates  to  the  connection  between  the  <,  0,  and  c  Orionis  nebulae.] 

I.  i  and  e  are  connected  by  two  nebulous  streaks : 

A.  The  co-ordinates  of  the  middle  of  the  first  are — 

J«_6S%  -72-,  -8o8,  -848,  -85%  -85%  -83%  -73-,  -46',  -28-; 
Ad—  630",  720",  810",  900",  990",  1170",  1260",  1350",  1530",  1620". 

B,  Those  of  the  second  are — 

Ja-348,     -i68,      +3",          +9%         +  i68,       +21"; 
A3  —  660",  —810",  —1180",  —  1 210",  —1360",  — 1650". 

Professor  SAFFORD  has  proposed  for  the  whole  system  the  name  Corona  Herschelii  ; 
D'ARREST  proposes  for  A  the  name  Semita  Bondiorum ;  the  name  Paeninsula 
Othonis  Struvii  is  proposed  for  a  region  there  described. 

II.  A  threefold  connection  of  6  and  c  Orionis  is  described — 

1.  Aa—  49%     —72%     —79s,     —  8i8,      —68s        —46"; 

J  5+430",  +720",  +900",  +1130"  +1440",  +1780". 

2.  The  middle  and  faintest  band  is  described. 

3.  The  3d  is  also  described.     The  name  Paeninsula  Cysati  is  proposed  for  the  region 

the  co-ordinates  pf  whose  middle  point  are  A  a  =  —  56",  A  d  =  +  490". 

;  See  DOBERCK. 

DAWES:  Mon.  Not.  R.  A.  S.,  viii,  p.  31.     [New  star.] 

DENNING  :  A.  N.,  Ixxx,  col.  299.     [Ten  stars  in  and  near  trapezium.]     See  SALTER. 

DE  Vico :  Mem.  Oss.  Coll.  Romano,  1839,  p.  31,  Plates  i  and  ii.     [Drawing;  new  stars  in  trapezium.] 

:  Same,  1840-41,  p.  22.     [Plate  by  RONDONI.] 

:  C.  R.,  xiii,  p.  449.     [Note  on  RONDONI'S  drawing.] 

DOBERCK  :  A.  N.,  xci,  col.  335.     [Remarks  on  COOPER'S  drawing.] 

:  Nature,  vol.  xvii,  p.  311.     [D'ARREST'S  work  on  nebulae.] 

DOPPELMAYER:  Himmels-Karten,  Blatt  26. 

ENGELMANN,  R. :  Messungen  90  Doppelsternen,  etc.,  p.  147.     [Variability  of  stars.] 

FAYE:  C.  R.,  vol.  Ix,  1865,  i,  p.  468.     [Remarks  on  SECCHI'S  observation  of  the  spectrum  of  the  nebula  in 

Orion.] 
FLAUGERGUES  :  C.  T.,  1802  (An  xi),  p.  361.     [Observations.] 

:  Mem.  de  PInstitut,  i  (An  vi),  1798,  p.  106. 

GILL:  Mon.  Not.  R.  A.  S.,  xxvii,  p.  315.     [Stars  within  the  trapezium  of  Orion.] 

GLEDHILL:  The  variable  (?)  star  h  No.  78,  near  the  trapezium  of  Orion.     Observatory,  1880,  p.  601. 

GOLDSCHMIDT:  A.  N.,  lix,  col.  31. 

HAHN,  VON:  B.  ].,  1797,  p.  157;  B.  J.,  1799,  p.  235 

[HALL]  :  Wash.  Ast.  Obs.,  1877,  App.  I.     Observations,  etc.,  of  S.  748. 

HERSCHEL,  J. :  Results  of  Astronomical  Observations  at  the  Cape  of  Good  Hope,  p.  25.     [With  a  plate.] 

:  Account,  etc.,  of  the  nebula  of  Orion.     Mem.  R.  A.  S.,  vol.  ii,  p.  487-     [with  plates.] 

:  Mem.  R.  A.  S.,  vol.  iii,  p.  189.     [Fifth  star  of  the  trapezium.]     See  also  same  vol.,  p.  187. 

HERSCHEL,  Capt.  J. :  Proc.  R.  S.,  vol.  xvi  (i867-'68),  pp.  417.  451-     [Observations  of  spectrum.] 
HERSCHEL,  W. :  MSS.  in  possession  of  Royal  Society,  London.     [Unpublished  observations.]     See  Mem. 

R.  A.  S.,  vol.  xxxv,  p.  52.     [There  is  an  erratum  in  the  Mem.  R.  A.  S.:  For  1780.  521  read  1780. 

134  and  for  22".4i  read  22". 521.] 
:  refers  to  the  nebula  of  Orion  in  P.  T.,  1782,  p.  129;  1785,  p.  258;  1789,  p.  249;  1791,  pp.  72> 

75,  77;  1811,  pp.  276,  320;  1814,  p.  258. 


I2  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

HOLDEN:  Mon.  Not.  R.  A.  S.,  vol.  xxxvii,  p.  231.     [List  of  drawings.] 

Washington  Astronomical  Obs.,  1874,  plate  vi,  fig.  4.     [Drawing.) 

Washington  Astron.  Obs.,  1877,  Appendix  I.     [Discussion  of  HALL'S  observations  of  2.  748.] 
HOOKE     Micrographia,  London,  1665,  p.  242.     [Discovery  of  the  4th  and  5th?  stars  in  trapezium.] 
HUGGINS  :  On  the  spectrum  of  the  great  nebula  in  Orion.     Proc.  R.  S.,  xiv,  1864,  p.  39 ;  also,  1865,  Jan.  26. 

Ditto,  ditto.     Proc.  R.  S.,  xx,  1872,  p.  379. 

Ditto,  ditto.     Proc.  R.  S.,  xxii,  1873,  p.  251. 

Mon.  Not.  R.  A.  S.,  xxvi,  p.  71.     [Nine  stars  in  trapezium.] 

Am.  Jour.^Sci.,  3d  series,  v,  p.  75. 

P.  T..^i868,  p.^541.     [Spectrum.] 


HUYGHENS  :  Systema  Saturnium.     4°.     1659.     [Drawing.]     See  KAISER. 

KAISER  F. :  Amster.  Tijdsch.  v.  Wiss.  en  Nat.  Wetens,  i,  1848,  p.  7.     [HUYGHENS'  drawing,  1694.] 

De  Sterrenhemel,  vol.  ii,  Plate  3,  pp.  538,  542.     [Original  drawing.] 
LALANDE:  Astronomic,  i,  p.  272.     [With  a  figure,  MAIRAN'S.] 
LAMONT  :  Ueber  die  Nebelflecken.     Munich,  1837.    4°.     [With  a  plate.] 
LAPLACE:  Exposition  de  la  Systeme  du  Monde,  p.  452.     [Opinion  that  nebulae  change.] 
LASSELL,  W. :  Observations  of  the  nebula  of  Orion,  etc.     Mem.  R.  A.  S.,  xxiii  (1854),  p.  53.     [Plate.] 

Proc.  R.  S.,  xvi,  p.  322.     [Measures  of  stars.] 

A.  N.,  xxxv,  col.  386. 

—     Mon.  Not.  R.  A.  S.,  xiv,  p.  74. 

Mon.  Not.  R.  A.  S.,  xvii,  p.  68.     [Relative  visibility  of  5th  and  6th  star  in  trapezium.] 

Mon.  Not.  R.  A.  S.,  xxii,  p.  164.     [New  star  in  trapezium.] 

Mon.  Not.  R.  A.  S.,  xxix,  p.  165. 

LEFEBVRE:  ROZIER  Obs.  sur  la  Physique,  xxii,  1783,  p.  34.     [With  drawing.] 

LEGENTIL:  Remarques  sur  les  Etoiles  ne"buleuses.     Hist,  de  1'Ac.  Roy.  des  Sciences,  1759,  p.  453  [with 

several  figures]. 
LESUEUR:  Proc.  R.  S.,  xviii,  pp.  i,  242.     [Spectrum.] 

:  Proc.  R.  S.,  xix,  p.  18.     [Spectrum.] 

LE  VERRIER:  C.  R.,  vol.  xliv,  1859,  pp.  1074,  1293-5.     [PORRO'S  new  star  in  nebula  of  Orion.] 
LIAPONOFF  :  See  STRUVE. 

:  Mon.  Not.  R.  A.  S.,  vol.  xxiii,  p.  228.     [Review  of  his  memoir.] 

LONG  :  Astronomy,  vol.  i,  p.  321,  Plate  67,  Fig.  96.     [Observations  and  drawing.) 

MAIRAN  :  Traite  de  1'Aurore  Boreale,  [p.  249;  nebula  Orionis  varies  in  shape ;  date  of  PICARD'S  drawing 

given  as  1673,  March  20.     MAIRAN'S  drawing  (Fig.  xxvii),  1727-1733.] 

MESSIER:  Nebuleuse  d'Orion.     Hist,  de  1'Acad.  R.  des  Sciences,  1771,  pp.  435,  458.     [Drawing.] 
NICHOL:  System  of  the  world,  1846,  p.  55.     [Lord  ROSSE'S  observations.] 
NOBILE  :  Osservazione  del  systema  748  2".     Rendiconte  d.  Ac.  d.  Sci.,  1877,  May,  No.  5. 
POND:  On  an  appearance  hitherto  unnoticed  in  the  nebula  of  Orion.     Mem.  R.  A.  S.,  iii,  1826,  p.  93. 

[Recession  of  the  nebula  from  the  stars.]     (See  also  same  volume,  p.  187,  for  an  observation  of 

J.  HERSCHEL  and  RAMAGE  on  this  point.) 
PORRO  :  Mem.  dell'Osserv.  Coll.  Romano,  i856-'57,  p.  3.     [Discovery  of  a  new  star  in  trapezium.] 

:  A.  N.,  xlvi,  col.  171.     [Same.] 

:  C.  R.,  xliv,  p.  1031.     [Same.] 

ROBINSON  :  Nature,  vol.  xv,  p.  292.     [Note  on  the  resolvability  of  the  central  part  of  nebula  Orionis.] 

RONDONI  :  See  DE  Vico. 

ROSSE  (Fourth  Earl  of) :  Account  of  observations  on  nebula  of  Orion,  1848-1867,  P.  T.;  1868,  part  i,  p.  57 

[Plates.]     For  a  review  of  this,  see  O.  STRUVE  in  V.  J.  S.,  1870,  p.  25. 

:  Mon.  Not.  R.  A.  S.,  xxix,  p.  165. 

SALTER  :  A.  N.,  Ixxx,  col.  299.     [Ten  stars  in  and  near  trapezium.] 

SCHMIDT  :  A.  N.,  vol.  xciii,  col.  78.     [Places  of  G.  P.  B.  Nos.  746,  784,  822.     822  is  certainly  variable;  at 

least  between  9.7  and  13  mag.] 
SCHROETER  :  Aphroditographische  Fragmente,  p.  248.     [Has  a  chart  and  memoir.] 

:  Beytrage  zu  den  neuesten  astron.  Entdeckungen,  vol.  iii,  p.  429.     [With  figures.] 

:  B.  J.,  1797,  p.  198.     [Observations.] 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


SCHROETER  :  B.  J.,  i8oi,  p.  128.     [Changes  in  nebula  of  Orion.] 

SECCHI     Mem.  dell'Oss.  Coll.  Romano,  i852-'56,  p.  80,  p.  92,  and  Plate  v.     [Drawing.] 

Mem.  dell'Oss.  Coll.  Romano,  i856-'57,  p.  3.     [New  star  in  trapezium.] 

Bull.  Meteor,  d.  Coll.  Romano,  1865,  January. 

Ace.  d.  Nuovo  Cimento,  serie  2a,  vol.  v-vi,  1872,  p.  20.     [The  solar  Corona  is  brighter  than  the 
nebula  of  Orion.] 

Atti  dell'Ac.  d.  N.  Lincei,  Anno  xxv,  sess.  iv,  1872,  p.  226.     [Spectrum.] 

Sulla  grande  nebulosa  di  Theta  Orione.     1868.     4°.     Mem.  Ital.  Soc.  Firenze,  vol.  i.     [Memoir; 
drawing;  spectrum.] 

A.  N.,  xlv,  col.  60.     [Sketch  of  nebula  of  Orion.] 

Mon.  Not.  R.  A.  S.,  vol.  xviii,  p.  8. 

Mon.  Not.  R.  A.  S.,  xxv,  p.  153.     [Spectrum.] 

Mon.  Not.  R.  A.  S.,  vol.  xxviii,  p.  162;  xxix,  p.  165. 

C.  R.,  xliv,;p.~i279,  and  xlv,  p.  170.     [PORRO'S  new  star.] 

C.  R.,  Ix,  pp.  460,^543.     [Spectrum.] 

C.  R.,  Ixv,  p.  63. 

C.  R.,  Ixvi,  p.  643.     1868.     [Spectrum,  etc.] 

Sugli  Spettri  Prismatici.     Mem.,  i,  ii,  iii. 
SENARMONT:  C.  R.,  xliv,  pp.  1075,  I294-     [PORRO'S  new  star.] 
SMITH  :  Opticks.     4°.     [HUYGHENS'  drawing.] 
STONE,  E.  J.,  and  CARPENTER  :  Mon.  Not.  R.  A.  S.,  xxiv,  p.  92.     [On  G.  P.  BOND'S  drawing  of  nebula  of 

Orion.] 

STRUVE,  O. :  Obs.  de  la  grande  nebuleuse  d'Orion,  avec  4  planches.     Mem.  de  1'Acad.  Imp.  des  Sciences 
de  St.  Petersbourg,  tome  v,  No.  4,  1862.     See  M.  M.,  ii,  p.  517.     [Abstract  of  above  memoir.] 

Bull,  de  la  Classe  Phys.-Math.  de  UAcad.  Imp.  de  St.  Petersbourg,  xvi,  1858,  col.  113. 

M.  M.,  iii,  p.  535.     [Observations  at  Malta.] 

M.  M.,  iii,  p.  550.     [Variability  of  nebula  of  Orion.] 

Mon.  Not.  R.  A.  S.,  xvii,  p.  225.     [Stars.] 

Bestimmung  d.  Constante  der  Praecession,  p.  40.     [Proper  motion  of  6  Orionis.] 

V.  J.  S.,  1870,  p.  25.     [Review  of  the  memoir  of  Lord  ROSSE.] 

W. :  Rapport  sur  les  observations  de  LIAPONOFF  sur  la  nebuleuse  d'Orion.     Bull,  de  la  Classe 
Phys.-Math.,  vol.  xii,  p.  316,  and  Melanges  Math.,  ii,  p.  45. 

Catal.  Nov.  Stell.  Duplic.,  1827,  p.  xiv.    [Discovery  of  5th  star  in  trapezium.]    Also,  p.  242.    [Sys- 
tem of  0  Orionis.] 
TEMPEL  :  A.  N.,  Iviti,  col.  240.     [Drawing.] 

:  A.  N.,  Ixxx,  col.  29.     [Trapezium.] 

:  Unpublished  drawing,  made  in  1876. 

TISSERAND  :  Bull.  Inter.  Obs.  Paris,  1876,  No.  119;  also,  C.  R.,  Ixxxi,  April  17,  p.  891. 
TROUVELOT  :  Annals  Harv.  Coll.  Obs'y,  vol.  viii.     [Drawing.] 

:  Wash.  Ast.  Obs.,  1874,  Appendix  I,  Plate  vi,  Fig.  4.     [Drawing.] 

Vico  :  See  DE  Vico. 

VOGEL,  H.  C. :  A.  N.,  Ixxviii,  col.  245.     [Spectrum.]     Also,  Bothkamp  Observations,  vol.  i,  p.  56. 

VON  HAHN  :  B.  J.,  1797,  p.  157. 

-:  B.  J.,  1799,  p.  235. 
WEBB  :  Intellectual  Observer,  vol.  xii,  p.  258.     [History.] 

:  Mon.  Not.,  R.  A.  S.,  xxvi,  p.  208.     [Account  of  his  drawings  and  observations.] 

WINLOCK  :  Astronomical  Engravings  from  the  Observatory  of  Harvard  College,  Plate  24.     [Drawing  of 

central  part  by  TROUVELOT.]     See  Annals  Harv.  Coll.  Obs.,  vol.  viii. 
WINNECKE  :  Melanges  Math.,  iii,  p.  499,  and  Bull,  de  1'Ac.  Imp.,  vii,  p.  18. 

:  Mon.  Not.  R.  A.  S.,  xxiv,  p.  7.     [New  stars  near  6  Orionis.] 

WOLF,  C. :  Sur  la  variabilite  des  nebuleuses.     Association  scientifique  de  France,  No.  535,  1878,  February 

3,  p.  277.     [History  of  the  nebula  of  Orion.] 
WOLF,  R. :  J.  B.  CYSAT  von  Luzern,  1853. 
:  A.  N.,  xxxviii.  col.  109. 


STRUVE 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


List  of  Telescopes  employed  to  observe  the  Nebula  of  Orion. 


Observer. 

Date. 

Telescope. 

Aperture, 
inches. 

Focus, 
feet. 

Maker. 

CYSAT      .     .     . 

1618 

Refractor    . 

? 

6,  10 

? 

HUYGHENS     .     . 

1656-94 

Refractor    . 

*-33 

12  or  23 

HUYGHENS. 

HOOKE 

1665 

Refractor    . 

3-5 

36 

HOOKE. 

PlCARD       .       .       . 

1673 

Refractor  ?  . 

p 

? 

? 

X 

MAIRAN    .     .     - 

i73i 

Refractor    . 

? 

7,  18,  22 

? 

LONG  .... 

1742 

Refractor    . 

? 

17 

LONG.? 

LCGENTIL       .     . 

1758 

Reflector    . 

? 

6 

? 

MESSIER  .     .     . 

1771 

Refractor    . 

3-33 

3-5 

DOLLOND. 

W.  HERSCHEL     . 

1774  to  1811 

Reflectors  . 

4  J  to  48 

5i,  7,  10,  20,  40 

HERSCHEL. 

LEFEBVRE      .     . 

1779 

Reflector    . 

? 

3i 

? 

SCHROETER      . 

1794-98 

Reflectors  . 

6  to  19 

7,13,27 

HERSCHEL,  SCHRADER, 

J.  HERSCHEL 

1824,  1837 

Reflector    . 

1  84 

20 

HERSCHEL. 

POND  .... 

1826 

Reflector    . 

. 

26 

RAMAGE. 

LAMONT    .     .     . 

1839 

Refractor    . 

10.5 

15 

FRAUENHOFER. 

De  Vico  .     .     . 

1839 

Refractor    . 

<* 

H 

FRAUENHOFER. 

KAISER     .     .     . 

1844 

Refractor    . 

6 

? 

? 

COOPER    .     .     . 

1847 

Refractor    . 

13-2 

25 

CAUCHOIX. 

LASSELL    .     .     . 

1847,  1854 

Reflector    . 

24 

20 

LASSELL. 

W.  BOND  .     .     . 

1848 

Refractor    . 

15 

23 

MERZ. 

LlAPONOFF      .       . 

1847 

Refractor    . 

10 

16 

MERZ. 

O.  STRUVE     .     . 

1851 

Refractor    . 

15 

23 

MERZ. 

SCHMIDT  .     .     . 

1861 

Refractor    J 

6 

6? 

DOLLOND. 

LASSELL   .     .     . 

1862 

Reflector    . 

48 

37 

LASSELL. 

SECCHI      .     .     . 

1862-68 

Refractor   . 

9.6 

M 

MERZ. 

WEBB  .... 

1863 

Refractor    . 

5 

5 

A.  CLARK. 

G.  P.  BOND    .     . 

1865 

Refractor    . 

15 

23 

MERZ. 

WEBB  .... 

1867 

Reflector    . 

9-4 

? 

WITH. 

BIRD    .... 

1866 

Reflector    . 

12 

? 

BIRD. 

Lord  ROSSE  . 

1867 

Reflector    . 

72 

55 

ROSSE. 

D'ARREST      .     . 

1872 

Refractor   . 

10.5 

15 

MERZ. 

TROUVELOT   .     . 

1874 

Refractor    . 

15 

23 

MERZ. 

TROUVELOT   .     . 

1875 

Refractor    . 

26 

32 

A.  CLARK  &  SONS. 

KEELER    .     .     . 

1878 

Refractor    . 

2.5 

? 

? 

H  OLDEN     .       . 

1874-80 

Refractor    . 

26 

32 

A.  CLARK  &  SONS. 

LANGLEY  .     .     . 

1879 

Refractor    . 

13 

? 

Made  by  Frrz  and  re- 

figured  by  A  .  CLARK. 

DRAPER    .     .     . 

1880 

Refractor    . 

ii 

? 

A.  CLARK  &  SONS. 

HOLD  EN   .     .     . 

1881 

Refractor   . 

15.5 

20 

A.  CLARK  &  SONS. 

I.— HISTORY   OF  THE  VARIOUS  RESEARCHES  ON   THE   NEBULA   OF   ORION   IN 

CHRONOLOGICAL   ORDER. 

The  drawings  and  memoirs  are  here  considered  in  chronological  order.  First  a 
wood-cut  of  the  original  drawing  is  given,  followed  by  extracts  more  or  less  copious 
relating  to  the  observations.  These  wood-cuts  were  made  in  the  following  way:  Good 
prints  of  the  original  drawings  were  selected  and  photographed  on  a  scale  of  one 
English  inch,  equal  to  the  distance  between  the  stars  Gr.  P.  B.  685  and  741;  the  scale 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  15 

is  thus  about  i  inch  —  1 29"  of  arc.  The  copying-lens  used  produced  no  appreciable 
distortion.  The  wood  cuts  have,  however,  never  been  used  as  evidence.  A  set  of 
photographic  prints  of  the  original  drawings  has  been  constantly  used  for  comparison 
and  for  suggestions  as  to  doubtful  points.  In  every  case  reference  has  been  made  to 
the  original  drawing  (when  possible),  or  to  the  original  engraving  or  wood-cut.  The 
negatives  so  made  were  used  to  transfer  the  photograph  on  to  wood,  and  the  various 
revises  of  the  cuts  have  been  again  compared  with  the  original  publication.  They 
are  therefore  tolerably  faithful  representations  of  their  originals,  and  will  serve  to  recall 
them  to  those  readers  who  have  not  these  originals  at  hand. 

As  they  are  nearly  all  on  the  same  scale,  and  as  the  drawings  made  by  reflectors 
have  been  inverted  so  as  to  present  the  appearances  as  seen  in  a  refractor,  they  are 
all  immediately  comparable  (except  a  few  of  the  figures  which,  for  special  reasons,  are 
on  other  scales),  and  they  are  interesting  on  account  of  the  enormous  differences  at 
once  apparent  even  among  the  more  modern  drawings. 

Everything  relating  to  the  Huyghenian  region  I  have  attempted  to  give  fully, 
generally  in  the  words  of  the  author.  Many  of  the  memoirs  contain,  beside  the 
records  of  original  observations,  a  discussion  of  the  results  obtained  by  others,  and 
these  I  have  usually  given  in  the  words  of  the  authors.  Thus  the  discussion  of  the 
various  drawings  is  partly  completed  in  the  progress  of  the  work.  I  have  resumed 
this  discussion  in  the  light  of  the  Washington  observations.  By  including  these  dis- 
cussions, the  admirable  resumes  of  'LIAPONOFF,  STRUVE,  D'ARREST,  and  others,  are 
available  for  immediate  reference. 


It  was  first  formally  pointed  out  by  Professor  WOLF,  of  Zurich,  that  HUYGHENS 
was  not,  as  commonly  supposed,  the  first  discoverer  of  the  nebula  of  Orion,*  but  that 
this  distinction  must  be  assigned  to  CYSAT  of  Luzern,  who  saw  it  in  1618,  38  years 
before  HUYGHENS  published  his  account  of  it.  It  is  now  just  visible  to  the  naked  eye 
under  good  circumstances,  but  it  does  not  appear  to  have  been  seen  by  any  of  the 
early  astronomers,  not  excepting  the  acute  ABDEL  RAHMAN  AL  SUFI,  who  was  acquainted 
with  the  nebula  of  Andromeda  before  A.  D.  950. 

CYSAT  speaks  of  his  discovery  as  follows : 

"  Caeterum  huic  phaenomeno  similis  stellarum  congeries  est  in  firmamento  ad 
ultimam  stellam  gladii  Orionis,  ibi  enim  cernere  est  (per  Tubum)  congestas  itidem 
aliquot  stellas  angustissimo  spatio  et  circumcirca  interque  ipsas  stellulas  instar  albae 
nubis  candidum  lumen  affusum."f  This  observation  of  CYSAT'S  is  mentioned  by 
BESSEL  in  the  Berliner  Jahrbuch,  1808,  p.  122. 

OBSEEVATIONS  OF  HTJYGHENS. 

Although  HUYGHENS  was  not  the  first  observer  of  the  nebula  of  Orion,  yet  his 
discovery  was  made  independently.  His  account  of  it  is  given  in  the  Systema  Sat- 

*  Astronomische  Nachrichten,  vol.  xxxviii,  col.  109,  and  "].  B.  CYSAT  von  Luzern."    4°.     Bern,  1853. 
t  CYSAT  Mathemeta  Astronomica  de  Cometa  Anni  1618,  p.  75. 


i6 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


urnium  (1659,  p.  8),  with  a  figure  which  I  have  reproduced  from  the  original  work.* 
HUYGHENS  says  that  in  examining  the  stars  of  the  sword  of  Orion  in  1 656  with  a  telescopef 

he  perceived,  instead  of 
the  three  stars  com- 
monly known,  twelve 
in  all,  of  which  three 
were  close  together, 
and  four  others  which 
shone  "as  if  through  a 
cloud,"  so  that  the  space 
around  them  appeared 
much  brighter  than  the 
rest  of  the  sky,  which 
was  serene  and  quite 
black,  so  that  the  ap- 
pearance was  produced 
of  looking  through  an 
opening.  All  this  he 
saw  many  times  still 
unchanged  in  the  same 
spot,  and  he  concludes 
that  this  wonderful 
thing,  whatever  it  may 
be,  has  there  a  place, 
apparently  forever;  the 
like  of  it  he  has  never 
seen  among  the  fixed 
stars.  It  will  be  ob- 
served that  this  figure,  in  the  original,  is  uniformly  shaded  throughout,  so  as  to  appear 
to  represent  a  uniform  brilliancy  in  the  nebula. 

It  appears  that  HUYGHENS'  observation  did  not  escape  HOOKE,  for  we  find  the 
following  reference  to  this  object  in  his  Micrographia.  For  this  reference  I  am  indebted 
to  the  courtesy  of  HENRY  B.  WHEATLEY,  esq.,  assistant  librarian  of  the  Royal  Society 
of  London,  to  whom  all  matters  relating  to  the  activity  of  HOOKE  are  familiar  through 
his  long  study  of  his  life. 

"In  that  notable  asterism  also  of  the  sword  of  Orion,  where  the  ingenious  Monsieur 
HUGENS  VAN  ZULICHEM  has  discovered  only  three  little  stars  in  a  cluster,  I  have,  with 
a  36-foot  glass,  without  any  aperture  [diaphragm]  (the  breadth  of  the  glass  being  about 
some  three  inches  and  a  half),  discovered  five,  and  the  twinkling  of  divers  others  up 
and  down  in  divers  parts  of  that  small  milky  cloud."! 

*  For  access  to  a  fine  copy  of  the  original  work  I  am  indebted  to  Professor  NEWCOMB. 

t  ARAGO  in  Annuaire  du  Bureau  des  Longitudes,  1842,  p.  268,  states  that  the  telescopes  of  HUYGHENS  of  12  and  23 
eet  had  an  aperture  of  2%  inches  (63  m.  m.),  and  magnified  48,  50,  and  92  diameters, 
t  Micrographia,  by  ROBERT  HOOKE,  folio,  London,  1665,  p.  242. 


FIG.  i.     HUYGHENS,  1656. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  17 

HOOKE'S  1  2  -foot  telescope  magnified  74  diameters,  according  to  AEAGO  (op.  cit., 
p.  269),  and  it  is  likely  that  even  a  higher  power  was  used  on  the  36-foot. 

Whether  HOOKE  really  saw  the  fifth  star  of  the  trapezium,  and  the  question  as  to 
how  faint  stars  can  be  seen  in  the  nebula  with  an  aperture  of  3  5  inches,  I  have  dis- 
cussed at  length  in  Washington  Astronomical  Observations  for  1877,  Appendix  I,  "On 
the  Multiple  Star  2  748,"  and  the  observations  are  given  at  length  later  in  this  work, 
The  conclusions  reached  there  are  that  from  HOOKE'S  language  in  the  Micrographia, 
as  well  as  from  an  entry  in  the  MS.  journal  of  the  Royal  Society  of  London  (which 
is  discussed  by  Sir  JOHN  HERSCHEL  in  Mem.  R.  A.  S.,  vol.  Ill,  p.  189),  no  other  stars  than 
the  fourth  and  fifth  stars  could  have  been  meant  by  HOOKE;  and  yet,  that  the  obser- 
vations made  with  the  26-inch  refractor  at  Washington,  with  its  aperture  reduced  to 
3.5  inches,  show  that  the  5th  star  cannot  now  be  certainly  seen  with  a  telescope  supe- 
rior to  HOOKE'S.  The  obvious  conclusion  from  these  two  facts,  if  accepted,  is  that 
the  fifth  star  is  now  fainter  than  in  HOOKE'S  day  (1666).  I  am  not  prepared  to  assert 
this,  although  I  know  of  no  way  of  explaining  HOOKE'S  early  observation  other  than 
that  which  I  have  stated.  HOOKE  must  have  had  the  original  of  Fig.  i  before  him, 
and  "  three  little  stars  in  a  cluster"  are  only  to  be  found  in  the  trapezium.  These 
three  HOOKE  declares  to  be  five. 

OBSERVATIONS  OF  HUYGHENS  (1694). 

In  an  account  of  HUYGHENS  by  KAISER*  is  a  description  of  HUYGHENS'  observa- 
tions on  this  nebula,  and  a  drawing  found  among  his  MS.  is  given,  which  we  reproduce 
in  Fig.  2. 

"On  the  8th  of  January,  1684,  he  first  perceived  that  the  group  of  stars  in  the 
nebula  of  Orion  in  which,  up  to  this  date,  he  had  only  been  able  to  make  out  three 
stars  with  difficulty,  was  composed  of  four.  Beside  this  note  we  find  in  HUYGHENS' 
journal  another  observation.  This  is  the  very  last  astronomical  note  made  by 
HUYGHENS. 

"  HUYGHENS  discovered,  in  the  year  1656,  the  nebula  in  the  constellation  of  Orion, 
which,  in  later  times,  has  been  productive  of  so  much  research  and  speculation.  That 
nebula  was  represented  by  him  in  his  Sy  sterna  Saturnium,  page  8,  and  is  there  exhib- 
ited as  a  spot  of  irregular  shape  over  which  the  light  is  equally  distributed.  HUYGHENS 
certainly  did  not  know  of  how  much  importance  a  correct  representation  of  the  nebula, 
made  in  his  time,  would  be  to  astronomers  in  later  years,  and  it  is  nearly  certain  that 
it  could  not  have  appeared  so  uniformly  illuminated,  or  with  such  distinct  edges  as  he 

has  indicated. 

"  Not  long  after  HUYGHENS  had  published  his  discovery,  a  new  representation  of 
the  nebula  was  given  to  the  world  by  PiCARDf  which  coincides  in  many  respects  with 
that  of  HUYGHENS,  but  which  ascribes  to  the  nebula  different  external  form.  A  century 
after  HUYGHENS,  LE  GENTIL  gave  two  illustrations  which  neither  coincided  with  each 
other  nor  with  the  representations  of  HUYGHENS  and  PICARD,  and  differing  widely  from 
the  beautiful  illustration  produced  a  fe^w^eari^ 


*  Tijdschrift  voor  de  Wis-sen  Naturkundige  Wetemchappen,  vol.  i,  1848,  p.  7. 
1  1  believe  this  was  never  published,  but  was  communicated  in  MS.  to  GODIN. 
App.  V  -  3 


i8 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


was  represented  for  the  first  time  as  flowing  and  spreading,  and  with  the  light 
unequally  distributed.  The  differences  between  all  these  drawings,  between  them- 
selves, as  well  as  with  the  later  efforts  of  SCHRCETER  and  HERSCHEL  seems  to  indicate 
that  the  nebula  underwent  great  variations ;  from  these  one  might  even  infer  a  doubt 
as  to  its  very  existence. 

"The  younger  HERSCHEL,  however,  did  not  allow  himself  to  be  misled  by  this 
seeming  contradiction.  He  knew  the  difficulty  of  producing  a  correct  representation 
of  such  an  object,  and  from  the  illustrations  given  by  his  predecessors,  which  were 
probably  made  with  imperfect  means,  he  came  to  the  conclusion  that  the  nebula  of 
Orion  had  undergone  no  considerable  change  since  its  discovery.  For  our  knowledge 
of  the  higher  regions  of  the  heavens  is  certainly  not  so  definite  as  to  assure  the  possi- 
bility of  indicating  undemonstrable  variations  in  the  nebulae ;  and  every  contribution 
which  can  serve  to  confirm  or  oppose  HERSCHEL'S  ideas  about  the  nebula  of  Orion 
may  be  considered  as  an  important  gain  to  science. 

"  Such  a  contribution  HUYGHENS  left  us  in  his  last  astronomical  note.  This  con- 
tains a  representation  of  the  nebula  of  Orion,  until  now  entirely  unknown,  drawn  with 
the  pen  in  his  journal  of  the  4th  of  February,  1694.  There  is  certainly  no  more 
intractable  instrument  than  a  writing  pen  for  giving  an  illustration  of  such  an  object, 
and  yet  the  drawing  of  HUYGHENS  expresses  to  us  something  of  the  form  of  its  most 
illuminated  portion  and  the  play  of  its  edges,  with  the  relations  of  the  surrounding 
stars.  Of  all  the  old  drawings  this  approaches  much  more  nearly  the  present  appear- 
ance of  the  nebula  in  Orion,  which  is  favorable  to  the  younger  HERSCHEL'S  opinion. 
HUYGIIENS  did  not  indicate  the  telescope  with  which  he  made  this  observation ;  but  it 
was  most  probably  with  that  of  44  feet  in  length,  which  he  had  had  constructed  a 
short  time  before  at  his  Hofwyk  station  at  (or  near)  the  Hague,  which  was  a  new  and 
most  suitable  instrument. 

"  We  can  be  very  certain  that  the  illustration  was  not  made  by  measurements,  and 

therefore,  to  make  the  matter  clear,  can  allow 
ourselves  a  slight  variation  between  the  present 
relative  position  of  the  stars  in  the  nebula  and 
their  position  as  given  by  HUYGHENS. 

"  The  illustration,  with  a  brief  note,  forms 
the  contents  of  the  last  page  of  HUYGHENS' 
journal.  We  think  we  are  subserving  a  not 
unimportant  use  in  presenting  a  very  exact 
reproduction  of  the  page,  in  order  to  make  this 
communication  complete." 

This  drawing  of  HUYGHENS  is  very  pre- 
cious, since  it  is  a  fac-simile  of  the  rough  sketch 
in  his  note-book,  untouched  by  the  engraver. 
On  it  we  find  the  trapezium  correctly  laid  down. 
The  Sinus  Gentilii  is  plainly  indicated;  the  Sinus  magnus  also,  and  the  three  stars  [Gr.  P. 
B.  685,  708,  741]  are  outside  the  light.  Even  the  true  north  edge  of  the  Sinus  magnus  is 
indicated,  while  the  boundaries  of  the  unshaded  portions  are  similar  to  the  appearances 


FIG.  2.     HUYGHENS,  1694. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  19 

seen  to-day  in  small  telescopes.  The  angle  between  the  frons  and  occiput  is  116°; 
in  LE  GENTIL'S  drawing  (i  758),  Fig,  6,  it  is  129°.  Its  value  to-day  is  about  91°.  The 
angle  between  the  occiput  and  the  north  side  of  the  Huyghenian  region  is  according 
to  HUYGHENS,  Fig.  2,  1 10°  ;  in  LE  GENTIL'S,  Fig.  6,  1 18°  ;  to-day  it  is  about  120°. 

OBSERVATIONS  OF  MAIRAN  (1731). 

In  Traite  de  TAurore  Boreale  (1733),  p.  248,  MAIRAN  notes  that  HUYGHENS  used 
telescopes  22^  Paris  feet  in  length,  and  that  HUYGHENS  declares  that  it  was  only  with 
such  that  the  nebula  of  Orion  was  well  seen.  MAIRAN  asserts,  however,  that  he  saw  it 
in  1733  with  a  7-foot  telescope,  from  which  fact  he  deduces  the  inference  that  it  is 
denser  [brighter]  than  in  HUYGHENS'  time.  "  Quant  a  sa  figure,  je  crois  aussi  qu'elle 
varie ;  et  c'est  qui  m'a  e'te'  confirm^  par  deux  astronomes  [MM.  GODIN  et  GRANDJEAN 
DE  FOUCHY]  que  j'avais  pi-ie*  d'y  regarder  avec  moi M.  GODIN  m'a  commu- 
nique* de  plus  un  dessiri  et  une  observation  manuscripte  de  M.  PICART  de  2Ome  mars 
1673,  ou  la  forme  exterieure  de  cet  espace  lumineux  differe  de  celle  de  M.  HUYGHENS." 
.  .  .  .  .  After  describing  the  position  of  the  nebulous  star  n.  f.  0  Orionis  [G.  P. 
B.  757],  MAIRAN  proceeds  (p.  249),  "la  figure  xxvii  represent  ces  objets  renverse's  et 
tels  qu'ils  m'ont  paras  le  plus  souvent  depuis  cinq  a  six  ans  avec  une  lunette  de  1 8  et 
de  22  pieds."  PICARD'S  (from  Hist,  del  'Acad.,  1759,  Plate  21,  Fig.  5),  and  MAIRAN'S 
figures  are  given  in  Figs.  3  and  4.  In  1 733  the  original  MS.  drawing  of  PICARD  was  in 
existence,  and  MAIRAN  refers  to  it  as  a 
proof  that  the  "  exterior  form"  differs  from 
that  given  by  HUYGHENS.  His  own 
figure  represents  the  "  exterior  form" 
uniformly  filled  with  nebulosity. 


FIG.  3.     MAIRAN,  1731. 


FIG.  4.     PICARD,  1673. 


20 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


OBSERVATIONS  OF  LONG  (1742). 

ROGER  LONG,  of  Cambridge,  author  of  "Astronomy,  in  five  books"  (1742),  was 

also  an  observer.  In  vol.  i,  p.  32 1  et  seq., 
[Plate  67,  Fig.  96],  we  find  his  account 
of  the  nebula  of  Orion:  "HuyGHENS,who 
first  discovered  this  wonderful  appear- 
ance, as  he  justly  calls  it,  has  given  us  a 
draught  of  it,  but  the  stars  are  all  drawn 
nearly  of  equal  magnitude,  and  the  lumi- 
nous space  is  more  defined  than  it  ought 
to  be;  which  faults  were,  in  all  likeli- 
hood, owing  to  the  mistakes  of  the  en- 
graver. I  have,  therefore,  in  figure  96, 
given  another  scheme  of  it,  such  as  I  have 
often  seen  it  through  a  telescope  of  17 
feet,  and  have  expressed  therein  the  ap- 
parent magnitudes  of  the  several  stars." 


FIG.  5.     LONG,  1742. 


OBSEKVATIONS  OF  LE  GENTIL  (1758). 

A  memoir  read  to  the  French  Academy  of  Sciences  July  26,  1 758,  by  LE  GENTIL, 
is  the  first  in  which  a  proper  attention  was  paid  to  the  observations  of  the  details  of  a 
nebula,  and  it  is  indeed  somewhat  surprising  to  remark  how  carefully  LE  GENTIL'S 
observations  were  made  and  how  caiitiously  his  conclusions  were  drawn.  He  had 
observed,  in  common  with  others,  that  the  representations  of  the  nebula  of  Orion  by 
HUYGHENS  and  PICARD  did  not  agree,  and  he  adduces  in  this  paper  evidence  to  show 
that  the  nebula  in  Andromeda  had  also  varied  in  brilliancy;  so  that  he  says,  "Y 
seroit-il  en  effet  arrive  quelque  changement,  ou  ne  pourroit-on  pas  attribuer  cette 
grande  difference  ....  aux  differentes  longueurs  des  lunettes  dont  on  se  sera 
servi.  .  .  .?"  "C'est  ce  qui  m'a  engagd  a  employer  les  plus  courtes  et  les  plus  lon- 
gues  lunettes  que  j'ai  pu  avoir  a  ma  disposition,  pour  comparer  ensemble  les  differens 
effets  que  j'en  pourrois  tirer." 

"Ainsi  nous  avons  tout  lieu  de  croire  que  la  plus  grande  partie  de  la  difference 
qu'on  trouve  entre  la  description  de  SIMON  MARIUS  et  celle  que  j'ai  faite  de  la  nebuleuse 
d'Andromede,  vient  de  la  diffurente  longueur  des  lunettes  dont  nous  nous  sommes  servis." 

LE  GENTIL  remarks  that  the  nebula  of  Orion,  as  seen  by  him  with  various  tel- 
escopes for  several  years,  also  varied  considerably  in  form,  and  he  gives  his  observations 
in  detail.  March  10,  1758,  he  observed,  in  company  with  M.  JOLY  and  M.  PINGRE, 
with  a  gregorian  telescope  of  6  feet,  and  this  observation  was  repeated  with  the  same 
telescope  on  the  3d  of  April.  The  drawing  [Fig.  6]  was  made  by  LE  GENTIL  from 
observations  with  this  telescope,  and  afterwards  compared  with  the  heavens.  . 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


21 


The  Sinus  Gentilii  is  there  laid  down  distinctly,  and  faint  nebulosity  is  described 
extending  towards  the  north.  This  is  the  Regio  Picardiana  of  HERSCHEL.  It  is  to  be 
remarked  that  LE  GENTIL'S  figure  is  not  equally  bright  throughout,  but  that  it  is 
brightest  along  the  following  side  of  the  Sinus  Gentilii,  along  the  frons  and  along  the 
north  shore  of  Sinus  magnus.  An  inspection  of  a  good  print  of  this  engraving  will 
show  that  in  the  meridian  of  the  star  [G.  P.  B.  708]  the  end  of  the  brighter  part  of 
the  frons  is  shown  [the  point  B  of  LIAPONOFP,  the  following  point  of  Q  in  the  index- 
chart],  and  that  the  trapezium  is  situated  on  nebulosity  of  the  same  kind  as  that  near 
the  northern  limit  of  his  drawing  round  star  [G.  P.  B.  479  ?],  which  nebulosity  he 
expressly  describes  as  "  faint."  So  much  is  at  once  evident,  on 'a  careful  inspection  of 
all  the  prints  I  have  been  able  to  see.  I  was  at  first  inclined  to  suppose  with  Sir 
JOHN  HERSCHEL  that  "  these  older  representations  are  mere  curiosities  and  present  no 
points  of  exact  resemblance,"  and  that,  therefore,  such  a  shading  as  I  describe  was 
merely  an  accidental  impression.  I  have  frequently  recurred  to  this  drawing,  not 
being  able  to  believe  that  when  so  careful  an  observer  as  LE  GENTIL  expressly  set 
about  discovering  whether  the  changes  in  the  older  drawings  of  this  nebula  were  not 
due  to  a  difference  in  the  telescopes  employed,  he  should  be  content  to  leave  so  extraor- 
dinary a  figure  of  it  as  his  witness  of  its  shape  in  his  day 

On  examination  of  his  figure  with  a  magnifying  glass,  the  explanation  of  the 
varied  brightness  of  different  parts  of  his  drawing  is  at  once  manifest.  He  attempts 
to  represent  three  grades  of  brilliancy,  according  to  his  own  account:  ist,  the  brightest 
central  portions;  2d,  the  fainter  northern  portions  near  the  star  [479?]  and  near  the 
Sinus  Gentilii;  and  3d,  the  black  background  of  the  heavens.  The  copper-plate  engraver 
working  most  probably  under  LE  GEN- 
TIL'S  own  eye,  has  chosen  to  represent 
the  brighter  nebulosity  by  wavy  parallel 
lines  about  a  sixtieth  of  an  inch  apart ; 
the  lines  run  completely  across  the  whole 
picture,  over  the  black  ground  of  the 
heavens  as  well  as  over  the  Huyghenian 
region.  The  nebulosity  which  LE  GEN- 
TIL  in  his  memoir  explicitly  declares  to 
be  "faint,"  the  engraver  represented  by 
putting  in  a  faint  black  line  between 
each  pair  of  the  waved  parallels,  thus 
diminishing  the  brightness  of  the  en- 
graving. Various  parts  of  the  central 
portion  not  explicitly  declared  by  LE 
GENTIL  to  be  faint,  are,  however,  shaded 
precisely  the  same  way,  and  these  parts 
are  in  general  those  which  to-day  are 
faint.  I  have  submitted  this  drawing 
to  experienced  engravers,  and  I  am 
informed  that  there  is  no  manner  of  doubt  but  that  the  engraver  of  this  drawing 


FIG.  6.     LE  GENTIL,  1758. 


22  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

intended  that  the  portions  in  question  should  be  fainter.  I  also  can  have  no  doubt 
but  that  the  astronomer  so  drew  them  in  his  sketch,  which  was  ''verified  at  the 
telescope,"  and  from  which  the  engraver  worked.  This  drawing,  so  considered, 
will  give  us  important  testimony,  and  it  extends  our  knowledge  of  the  central  por- 
tion of  the  nebula  back  from  MESSIER  (1771),  formerly  considered  the  first  trust- 
worthy drawing,  to  1758.  It  has  been  said  that  LE  GENTIL  gives  PICARD'S  figure 
also,  and  it  will  be  found  on  a  similar  examination  of  PICARD'S  drawing  that  a  por- 
tion of  the  Eegio  sulmebulosa  is  on  that  figure  represented  as  fainter  by  the  same 
conventional  sign,  and  professional  engravers  assure  me  (as,  indeed,  any  one  can  con- 
vince himself  by  the  use  of  a  magnifier),  that  this  is  not  accidental,  but  designed 
HUYGHENS'  figure  is  likewise  given  by  LE  GENTIL,  but  this  is  represented  of  equal 
brilliancy  throughout,  as  was  done  in  HUYGHENS'  original  plate.  It  thus  becomes 
necessary  to  examine  the  evidence  with  regard  to  the  drawing  of  PICARD.  I  consider 
these  points  quite  important.  To  see  how  conclusive  the  proof  is  an  examination  of 
these  figures  should  be  made  with  a  magnifier.  The  original  drawing  of  PICARD  was 
made  March  20,  1673,  and  was  communicated  with  a  manuscript  observation  by 
GODIN  to  MAIRAN  in  1731,  or  about  that  time.  No  account  is  given  of  the  "  observa- 
tion," but  MAIRAN  refers  to  this  figure  (but  does  not  reproduce  it),  which  he  adduces  to 

prove  that  "  la  forme  exterieure"  is  different  from  his  own. 
MAIRAN'S  own  drawing  is  uniformly  shaded  throughout. 
LE  GENTIL  speaks  of  MATRAN'S  copy  of  PICARD'S  figure, 
as  follows:  "M.  MAIRAN  se  fonde  encore  sur  un  dessin 
de  M.  PICARD,  dans  lequel  la  forme  exterieure  de  cet 
espace  lumineux  differe  assez  de  celle  que  M.  HUYGHENS 
nous  a  laissee  du  meme  espace."  In  the  explanation 
of  his  various  figures  LE  GENTIL  says,  Fig.  5:  "Nebu- 
leuse  d'Orion  suivant  M.  PICARD."  There  is  nothing 
said  of  borrowing  the  figure  or  original  drawing  from 
MAIRAN.  It  is  impossible,  at  this  day,  to  decide  whether 

FIG.  7.     Drawing  made  at  Washington       Lfi  GENTIL  had  aCCCSS  to  the  Original  drawing  of  PiCARD 
through  tourmaline  plates.  ,  111         riiii 

or  not  My  own  idea  is  that  he  had.  I  shall,  however, 

treat  the  drawing  of  PICARD  in  both  ways:  ist,  as  if  MAIRAN'S  figure  was  correct; 
2d,  as  if  LE.  GENTIL'S  figure  (including  the  fainter  portions)  was  so.  In  this  way  we 
may  arrive  at  some  conclusion. 

"  N(ibuleuse  d'Orion  telle  que  je  1'ai  vue  le  10  mars  et  le  3  avril  1 758,  an  soir  avec 
un  telescope  de  6  pieds  de  longueur ;  1'ouverture  A  B  c  m'a  paru  de  65  degres  environ, 
et  les  trois  etoiles  inferieures  paroissent  faire,  avec  le  cott?  cD,  un  angle  de  pros  de  40 
degres."  The  angle  which  LE  GENTIL  called  about  40°  is  about  50°  to-day.  The 
stars  of  this  drawing  are  [G.  P.  B.,  Nos.  741,  708,  685,  9,  969,  and  479 ??]. 

During  1877  I  made  a  number  of  sketches  of  the  nebula  through  tourmaline 
plates,  so  as  to  reduce  the  light  at  will.  One  of  these  is  given  in  Fig.  7  (see  my  obser- 
vations of  1877,  February  3).  One  made  1877,  January  30  (not  given  here),  strikingly 
resembles  Fig.  6,  with  one  exception,  viz,  Fig.  6  puts  the  trapezium  on  darker  ground ; 
my  drawing  puts  it  on  brighter  ground. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  23 

If  we  regard  PICARD'S,  HUYGHENS',  and  LE  GENTIL'S  drawings  as  simple  evidences 
of  the  exterior  shape  of  the  nebula  in  their  times  (1656,  1673,  and  1758),  we  can  best 
examine  MAIRAN'S  idea  of  a 
change  in  ula  forme  exte- 
rieure" by  superposing  these. 
Fig.  ^8  shows  the  result  of  a 
superposition  of  the  three 
drawings.  The  true  posi- 
tions of  the  stars  are  denoted 
by  a  dot  surrounded  by  cir- 
cle; PICARD'S  positions  by  a 
dot  and  a  square;  HUYGHENS' 
by  a  dot  and  a  circle  of  dots ; 
LE  GENTIL'S  by  a  dot  and  a 
triangle.  HUYGHENS'  outline 
is  given  by  a  dotted  line; 
PICARD'S  by  a  broken,  and 
LE  GENTIL'S  by  a  full  line. 

If  we  correct  the  distor- 
tion of  each  drawing  by 
means  of  the  true  positions 
of  the  stars;  L  e.,  if 
we  suppose  that 
those  portions  of 
the  nebula  near  a 
star  are  correctly 
drawn,  while  por- 
tions distant  from 
stars  are  distorted 
by  the  same  propor- 
tional amounts  as 
the  star  positions, 
then  we  shall  have 
outlines  like  Fig.  9. 
In  this  figure  it  is 
seen  that  HUY- 
GHENS* and  PICARD 
agree  as  to  "la 
forme  exterieure," 
as  we  should  ex- 
pect, since  their 
telescopes  were,  in 
all  likelihood,  sim- 


FIG.  8.    Comparison  of  the  drawings  of  HUYGHENS,  PICARD,  and  LE  GENTIL, 

(uncorrected). 


FIG.  9.     Comparison  of  the  drawings  oi  HUYGHEXS,  PICARD,  and  LE  GENTIL  (corrected). 


*  A  comparison  of  Fig.  i  and  Fig.  9  shows  that  it  is  probable  that  HUYGHENS  began  his  drawing  of  1656  t 
the  right  hand  side  of  the  paper  he  was  using. 


24  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

ilar.  The  bounding  lines  of  LE  GENTIL  do  not  agree  with  the  others.  His  telescope 
was  quite  different  in  power  from  that  of  HUYGHENS. 

On  Fig.  9  I  have  also  put  the  bounding  lines  between  the  faint  and  bright  por- 
tions of  the  nebula,  as  given  by  PICARD  (broken  line  with  two  dots)  and  LE  GENTIL 
(full  lines,  faint).  There  is  no  marked  agreement,  but  it  is  to  be  noted  that  PICARD, 
thus  corrected,  agrees  even  more  closely  than  before  with  both  of  HUYGHENS'  figures. 

I  have  not  included  HUYGHENS  1694  and  MAIRAN  1731  in  these  comparisons,  as 
they  would  yield  no  additional  evidence,  but  simply  confuse  the  drawing. 

OBSERVATIONS  OF   MESSIER  (1771). 

The  observations  and  drawing  of  MESSIER  are  thus  described  by  himself: 

"  Le  dessin  de  la  n^buleuse  d'Orion,  que  je  pr^sente  a  I'Acade'mie  a  e'te'  trace"  avec 
le  plus  de  soin  qu'il  m'a  &t£  possible.  La  ne"buleuse  y  est  represented  telle  que  je  1'ai 
vue  plusieurs  fois  avec  une  excellente  lunette  achromatique  de  trois  pieds  et  demi  de 
foyer,  a  triple  objective,  portant  40  lignes  d'ouverture  [3.33  Paris  inches]  qui  grossis- 
soit  68  fois.  Cette  lunette  [a  e'te']  faite  a  Londres  par  DOLLOND.  J'ai  examine  cette 
ndbuleuse  avec  la  plus  grande  attention  par  un  ciel  entitlement  serein  :  savoir, 

Les  25  et  26  feVrier  1773  ;  Orion  au  meridien. 

Le  19  mars  entre  8  and  9  heures  du  soir. 

Le  23  entre  7  and  8  heures. 

Les  25  et  26  du  meme  mois,  a  la  meme  heure. 

Ces  observations  combiners  et  les  dessins  rapproche's  les  uns  des  autres,  m'ont  mis  a 
meme  de  rendre  avec  soin  et  precision  sa  forme  et  ses  apparences. 

Ce  dessin  servira  a  reconnoitre,  dans  la  suite  des  temps  si  cette  nebuleuse  est 
sujette  a  quelques  changemens.  II  y  auroit  deja  lieu  de  le  presumer :  car,  si  on 
compare  ce  dessin  avec  ceux  donne's  par  messieurs  HUYGHENS,  PICARD,  MAIRAN  et  avec 
LE  GENTIL  on  y  trouvera  un  changement  tel  qu'on  auroit  peine  a  se  figurer  que  ce  fiit 
le  meme.  ***** 

Le  jet  de  lumiere,  dirige  de  1'etoile  no.  8  a  J'etoile  no.  9  passant  a  cote*  d'une 
petite  e'toile  de  la  dixieme  grandeur,  t'toit  extremement  rare,  ainsi  que  la  lumiere  dirigee 
vers  1'etoile  no.  10  et  celle  qui  y  est  opposee  oil  sont  les  huit  etoiles  contenues  dans  la 
nebuleuse:  ******* 

*  M4m.  de  VAcad.  dca  Sci.,  1771,  p.  458. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION 


FIG.  10.     MESSIER  (1771). 


It  may  be  remarked  of  this  figure  that  the  star-positions  are  quite  erroneous,  and 
therefore  the  shape  of  the  nebula  as  a  whole  is  somewhat  distorted.  Unfortunately 
MESSIER'S  erroneous  star-positions  not  only  affect  his  own  drawing,  but  those  of 
SCHROETER  (1794)  also,  who  took  MESSIER'S  stars  as  a  basis  for  his  own  drawing. 

The  synonyms  of  some  of  MESSIER'S  stars  are  given  below : 


MESSIER'S 
Number. 

BOND'S 
Number. 

Remarks. 

MESSIER'S 
Number. 

BOND'S 
Number. 

Remarks. 

i 

467 

6 

•708 

Star. 

49- 

Not  numbered. 

Star. 

724 

Not  numbered. 

2 

505 

7 

734 

Star. 
Star. 

449 
479 

SOut  of  place  in  R.  A. 

S 
Star. 

741 
784 

Not  numbered. 

Star. 

523 

Star. 

822 

Not  numbered. 

4 

570 

9 

843 

5 

685 

10 

848 

Apr.  V- 


2t> 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION, 


OBSERVATIONS  BY  SIR  WILLIAM  HERSCHEL  (1774  TO  1811). 

• 
(From  the  original  papers  in  possession  of  the  Eoyal  Society,  referred  to  l)y  Sir  J.  F.  W. 

Herschel  in  vol.  154  of  the  Phil.  Trans.,  p.  6,  No.  2.) 

[These  papers  were  kindly  put  at  my  disposition  by  Sir  GEORGE  ATRY  and  by 
Professor  STOKES,  secretaries  of  the  Royal  Society,  during  a  visit  to  London  in  1876] 

"  1 7  74,  March  4.  [Journal  No. 
i .]  Saw  the  lucid  spot  in  Orion's 
sword  through  a  5^  feet  re- 
flector; its  shape  was  not  as  Dr. 
SMITH  has  delineated  in  his 
Optics,  though  something  resem- 
bling- it,  being  nearly  as  follows: 
From  this  we  may  infer  that 
there  are  undoubtedly  changes 
among  the  fixed  stars,  and  per- 
haps, from  a  careful  observa- 
tion of  this  spot,  something  might 
be  concluded  concerning  the 
nature  of  it. 

5.  Nov.  u,  [17] 76  The  lucid 
spot  in  Orion.  The  figure 
[see  page  2  7]  is  only  drawn 
for  the  description.  The 
greatest  glare  is  immedi- 
FIG.  u.*  HERSCHEL  (i 774).  ately  about  the  four  small 

stars.     The  3  stars  i,  3,  4,  were  almost  (on  the  upper  side  of  this  figure)  free 
from  any  glare.     There  was  a  total  darkness  in  the  corner  by  the  4  small  stars. 
The  stars  i,  2  [685,  628],  were  of  one  size, 
those          3,  4  [708,  741],  of  the  next, 
the  two      5,  6  [640,  619],  considerably  less, 

7  [669]?  very  small, 

8  [624],  rather  smaller  still. 

Instrument,  lo-feet  reflector;  power  only  120,  but  very  distinct. 
6.     Jan.  25th,  [17]  78.     See  figure,  page  6.     A  tollerable  (sic)  exact  delineation  of  the 
h          h  lucid  spot      At  the  eastern  side  the  rays  seem  to  make  an  equilatteral  (sic) 
triangle  with  the  stars  i  and  3  [685,  708].     It  goes  on  in  the  direction  i,  3,  4 
[685,  708,  741],  rather  approaching  to  4  [741]  ;  at  4  [785]  it  bends  round 
in  an  angle  of  about  110°  or  120°  towards  the  east. 

From  two  [628]  to  7  [669]  the  lucid  part  is  concave;  the  concave  part  being 
turned  towards  3  [708].  It  goes  to  the  northward  about  ^  of  the  distance  from  2 
to  7  beyond  7.  From  thence  it  turns  to  the  west  in  an  angle  of  about  75  or  70  deg. 

*Thie  figure  is  not  given  in  Journal  No.  i,  but  a  reference  is  there  made  to  the  Phil.  Trans,,  1811,  from  which  the 
cut  is  copied. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  27 

6.  Jan.  26,  [i7]78      6,  2,  i  [619,  628,  685],  make  a  straight  line  [so  in  1879].  • 

6,  8,  7  [619,  624,  669],  make  a  straight  line  [not  so,  1879]. 
4,  5,  8  [741,  640,  624],  make  a  straight  line  [so  in  1879]. 
The  lines      2,  5  [628,  640],  and  i,  3,  4  [685,  708,  741]  diverge. 
5  [640],  a  little  larger  than  6  [619]  [so  in  1879] 

7.  Feb.  7,  [i7]78.     The  situation  of  the  4  stars  in  the  lucid  spot  agrees  perfectly 

well  with  the  observation  of  Jan.  25. 

? 25,  [i  7] 78.  No  change  in  the  situation  of  those  4  stars  to  be  observed.     [Paper 

torn  here  so  that  the  date  is  illegible']. 

7      Dec.  15,   [i7]78,     6,  8,  7  [619,  624,  669],  a  straight  line  [not  so,  1879] 

6,  2,  i  [619,  628,  685],  a  straight  line  [so  in  1879]. 
4,  5,  8  [741,  640,  624],  a  straight  line  [so  in  1879]. 
The  lines  2,  5  [628,  640]  and  i,  3,  4  [685,  708,  741]  diverge. 

This  agrees  exactly  with  Jan.  26,  but  there  is  a  visible  alteration  in  the  figure  of 
the  lucid  part. 

28.  Oct'r  7,  [17]  79.  0  Orionis  see  Fig.,  p.  6.  The  line  6,  2,  i  [619,  628,  685]  is  a 
little  convex  towards  5  [640],  when  that  line  is  taken  into  the  middle  of  the  field; 
this  I  mention,  as  it  is  possible  there  might  be  a  little  curvature  arising  from  the 
spherical  figure  of  the  eye-glass,  tho'  I  believe  there  is  not.  If  a  line  be  drawn 
from  6  [619]  to  7  [669],  the  star  8  [624]  stands  outwards,  I  suppose,  no  less 
than  15°,  so  that  6,  8,  7  [619,  624,  669]  is  concave  towards  the  side  i,  3,  4 
[685,  708,  741]  [so  in  1879]. 

The  line  4,  5,  8  [741,  640,  624],  I  cannot  very  well  compare,  being  rather  too 
far  distant  by  the  power  I  now  use,  but  I  believe  it  is  not  far  from  a  right  line.  I  see 
a  9th  star,  which  is  marked  in  the  annexed  figure.  [This  figure  gives  i  =  685 ; 
2=628;  3  =  708;  4=741;  5  =  640;  6  =  619;  7  =  66958  =  624;  9  =  635.  Thenum- 
bers  of  three  figures  are  BOND'S.] 

619  =  6  2  =  628  where,  however,  not  the 

•—  ""•  i  =  685  least     exactness    is     in- 

tended. 
624  —  «•      / 

0  5  =  640  .3  = 

•4  =  74i 
635=9          / 

=  669 

Altitude  about  26  degrees,  14"  10'.    The  figure  of  the  lucid  part  is  very  much  altered 
43.    Dec.  5,  [i;]79.    6,  2,  i  [619,  628,  685]  concave,  vid.  page 6 and  28.    The  concave 
part  turned  to  the  south.     8,  6,  7  [624,  619,  669],  still  make  an  angle  at  6 
[619],  tho'  very  small.     I  see  the  9th  star  [635]  mentioned  page  29. 
45      Jan    22,   1780,  ioh  30'.     The  stars  6,  i,  2   [119,  628,  685]  instead  of  seeming 
concave  towards  the  north  appear  convex.     This  may,  however,  be  a  decep- 
tion, as  the  star  2  [628]  is  the  largest,  and  since  there  is  a  pretty  strong  aberra- 
tion on  account  of  a  fog,  its  diameter  is  more  encreased  than  that  of  i9J, 
and,  consequently,  may  give  the  balance  towards  the  north. 


28  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

46. '  Feb.  19,  1780.     Exactly  as  described,  page  28. 

46.  Feb  28,  1780.  The  two  stars  6,  2  [619,  628],  page  6,  measure  22  —  20^  = 
20%  parts  =  1 27/.8 12  The  measure  is  pretty  narrow,  but  I  believe  true  enough. 
The  two  stars  2,  5  [628,  640]  measure  24^  —  i%  =  I5".2o8  —  .937  ~  I^'AJI. 
This  is  also  a  pretty  narrow  but  just  measure.  The  two  stars  6,  8  [619,  624] 
measure  about  16—  i%  =.  14^  =:9".o62.  But  this  is  doubtful  on  account  of 
the  obscurity  of  the  star  8  [614]  which  is  hardly  perceptible  when  the  field  of 
view  is  illuminated  properly  so  as  to  make  the  parallel  hair  very  distinct.  The 
two  stars  5/8  [640,  624]  measure  34^  —  \%  —  21.458  —  .937  —  2o".52i.  This 
is  also  doubtful  on  the  same  account.  [N.  B.  A  new  reduction  of  these  measures, 
with  the  value  i  part  =  o".625i,  shows  no  change  to  be  required  in  the  hundredths 
of  seconds. ~\ 

8 1.  Oct.  10,  1780.  The  upper  stars  concave  by  the  hair.  The  spot  extremely  fine. 
The  4  stars  all  full,  round,  and  well  defined. 

84.  Nov.  24,  1780.  I  view  the  nebula  in  the  highest  perfection.  I  perceive  not  the 
least  change. 

319.  Jan.  31,  1783.  The  nebulous  part  is  quite  different  from  what  it  was  last  year. 
The  9th  star  [635]  very  strong,  the  nebula  about  it  and  the  7th  (669)  being 
much  dispersed. 

432.     Sept.  20,  1 783.     Has  evidently  changed  its  shape  since  I  saw  it  last,  20  ft,  200. 

441.     Sept.  28,  1783.     Nebula  in  Orion  is  surprisingly  changed. 

457.  15  Sw.    Nov.  3,  1783.    The  nebula  is  beautiful,  and  I  see  several  circumstances 

which  I  never  observed  with  other  instruments.  Just  close  to  the  4  stars  it  is 
totally  black  for  the  short  space  of  a  few  seconds.  In  the  open  black  part  of  the 
nebula  is  a  small  distinct  nebula  of  an  extended  shape  [o  TT  of  the  Index-Map}. 
The  eastern  branch  of  the  great  nebula  extends  very  far ;  it  passes  between 
two  very  small  stars,  and  runs  on  till  it  meets  a  very  bright  star.  The  nebulous 
star  below  the  nebula  is  not  equally  surrounded,  but  most  nebulous  towards 
the  south.  On  the  north  of  this  lesser  nebula  is  another  smaller  nebula  joining 
to  it,  which  is  much  fainter  than  the  other,  and  it  makes  a  rectangular  corner 
by  its  meeting  with  the  nebula  surrounding  the  star.  This  faint  nebula,  as 
well  as  that  contained  within  the  dark  part  of  the  great  one,  other  instruments 
did  never  show,  and  I  suppose  is  not  visible  by  them. 
296.  Sw.  Oct.  1 6,  1784.  The  beginning  of  the  nebula.  5  Monocerotis,  p.  41'  6"  n  o° 

^43'-     R-  A.  5h  23'  7",  P.  D.  95°  30'. 

Sweeping  Journal  No.  2.  Dec.  20,  1 784.  20  ft.  The  nebula  as  described,  but  moon- 
light, and  therefore  seemed  to  take  hardly  3A°  in  extent. 

Sweeping  Journal  No.  3.  Feb.  13,  1785.  I  examined  the  nebula  in  Orion  with  a 
new  10  ft.  reflector,  and  with  long  attention  could  just  perceive  my  small,  faint 
nebula  in  the  dark  part  of  the  great  nebula. 

458.  Sw.     Oct.  5,  1785.     A  wonderful  phenomenon. 

510.  Sw.  Jan.  1 8,  1786.  The  nebula  observed.  The  place  brought  to  the  present 
from  the  Connoiss.  des  terns  is  R  A.  5h  24'  49",  P.  D..  95°  33'  (var.  from  1769 
to  1786  in  R.  A.  49".8,  in  P.  D.  i'  i".4). 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  29 

528.     Sw.     Feb.  23,  1786.     The  4  stars  in  the  great  nebula  Orionis.     36  (v)  Orionis 

f.  3'  n",  n  i°  56',  R.  A.  5h  24'  42",  P.  D.  95°  32'. 
640.     Sw.     Nov.  28,  [17]  86.     The  nebula  which  I  saw  by  the  front  view  was  so 

glaring  and  beautiful  that  I  could  not  think  of  taking  any  place  of  its  extent. 
Rev.     Jan.  14,  1801.     Large  X-foot  telescope,  power  1 20.     As  before  described. 
Rev.     Feb.  n,  1806.     Large  10  feet.     The  4  stars  are  completely  in  the  nebulosity. 
The  3  stars  are  intirely  [sic]  out  of  it  with  270.     With  the  double  glass  appear- 
ances are  very  different. 
Rev.     Feb.  4,  1810.     10  feet.     The  nebulosity  is  intirely  [sic]  of  the  milky  kind,  and 

extends  a  great  way. 

Rev.  Dec.  31,  1810.  10  feet,  double  eye-piece.  The  4  stars  are  within  the  nebu- 
losity. The  star  No.  7  [669]  (see  the  figure  of  Oct.  7,  1779)  is  upon  the 
borders  of  the  dark  vacancy.  I  see  No.  9  [635]  very  well.  The  little  star 
between  3  [708]  and  4  [741]  is  still  within  very  faint  nebulosity.  [This  refers 
to  7  24  possibly,  but  I  do  not  understand  it.] 

The  nebulosity  reaches  beyond  4  [741]  as  far  as  from  i  [685]  to  4  [741]  nearly 
It  touches  a  very  small  star  [793 1],  and  from  that  star  goes  on  to  two  very  bright 
ones  [843,  905!]  in  the  direction  from  the  small  star  [793?]  to  the  preceding  one 
[843!]  of  the  two.  The  black  space  near  the  4  stars  is  much  contracted.  The  neb- 
ulosity from  i  to  4  is  concave,  the  concavity  being  to  the  following  side.  The  parallel 
is  nearly  in  the  line  of  i,  3,  4  [685,  708,  741].  I  can  see  8  different  condensations, 
notwithstanding  the  moon  is  very  bright.  The  nebulous  star  [734]  is  pretty  equally 
involved.  It  has  the  appearance  of  a  star  shining  through  a  very  faint  mist.  The 
star  is  a  little  larger  than  4  [741].  The  concavity  from  2  [628]  to  7  [669]  goes 
beyond  7  [669]. 

Rev.     Jan.  19,  1811.     10  feet.     2  of  the  4  stars  are  within  the  nebulosity.     No.  7 
[669]  is  very  near  the  borders  of  the  black.     The  little  star  [724]  between  and 
following  3  and  4  [708,  741]  is  still  within  very  faint  nebulosity.     The  nebu- 
losity reaches  beyond  4  [741]  rather  farther  than  from  i  to  4  [628  to  741]. 
X  feet.     I  perceive  7  or  8  different  condensations.     The  place  near  the  4  stars  is 
much  contracted.     The  nebulous  star  is  exactly  what  we  might  expect  to  see  if  a  star 
were  to  shine  through  whitish  nebulosity. 

40  feet.     5h  1 6',  B  affected. 

17',  B  much  affected. 

22',  the  4  stars  are  intirely  involved  in  nebulosity. 
The  7th  [669]  and  9th  [635]  stars  are  very  bright. 

In  the  brightest  part  are  four  places  brighter  than  the  rest.  I  see  the  small 
detached  nebula ;  it  is  extremely  faint.  It  is  between  the  corner  and  a  small  star.  The 
star  called  nebulous  is  within  a  nebulosity  nearly  detached;  but  the  small  stars  marked 
nebulous  in  the  figure  of  the  4th  of  March,  1774,  are  free  from  nebulosity.  There  is 
a  very  small,  nearly  detached  nebulosity  north  of  the  nebulous  star.  The  nebulous 
star  has  some  resemblance  to  a  star  shining  through  a  very  thin  mist 

[What  follows,  I  suppose  to  be  in  Sir  Wm.  HerscJieTs  handwriting.  I  suppose  what 
precedes  to  have  been  written  by  his  sister.] 


3Q  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Rev.     Mar.  13,  1811.     7  feet;  double  eye-piece.     The  following  or  rather  the  southern 

branch  (for  I  find  the  parallel  nearly  in  the  line  i,  2,  3  [685,  708,  741])  goes 

towards  the  preceding  star  e  [843]  of  the  two  large  stars  d  e  [905,  843],  or  rather 

a  little  preceding  it,  but  it  partly  includes  the  star  e  and  makes  it  appear  a  little 

nebulous.     The  light  about  the  nebulous  star  b  [734]  is  a  little  denser  nearer 

the  star  than  at  a  distance.     A  line  from  5  [640]  through  7  [669]  goes  to  b  [734], 

or  rather  a  little  south  of  it,  and  7  is  about  ^  of  the  distance  towards  b. 

The  star  [724]  south  of  3   [708]  and  4  [741]  makes  an  equilateral  triangle  with 

them.     The  two  large  stars  d  and  e  [905,  843]  are  parallel  to  i,  3,  4  [685,  708,  741], 

nearly.     A  line  from  the  4  stars  parallel  to  i,  3,  4  passes  a  little  south  of  the  small, 

formerly  nebulous,  star  c  [848].     There  are  many  other  stars  connected  with  the 

nebula  which  I  do  not  notice. 

Rev.  Mar.  15,  1811.  7  feet ;  double  eye-piece.  The  northern  branch  is  parallel  to 
the  stars  ab  [543,  734].  The  nebulosity  reaches  nearly  up  to  the  stars  g,  h 
[570,  523]- 

A  very  faint  nebulosity  still  joins  the  star  b  [734]  to  the  northern  branch,  but  b 

is  more  nebulous  than  the  intermediate  nebulosity.     The  southern  nebulosity  goes 

towards  the  star  e  [843],  and  some  part  of  the  very  faint  nebulosity  incloses  the  star. 

Rev.     Mar.   16,    1811.     10  feet  reflector.     Power,   100.     The  stars   i,  3  are  in  the 

parallel;  4  is  a  very  little  south  of  their  parallel.     The  nebulosity  about  b  [734] 

is  brightest  about  the  star. 

In  Philosophical  Transactions,  1802,  p.  499,  Sir  WILLIAM  HERSCHEL  alludes  to 
changes  in  the  nebula  of  Orion,  as  follows: 

"IX.   Of  Milky  Nebulosity. 

11  The  phenomenon  of  milky  nebulosity  is  certainly  of  a  most  interesting  nature ; 
it  is  probably  of  two  different  kinds ;  one  of  them  being  deceptive,  namely,  such  as 
arises  from  widely-extended  regions  of  closely  connected  clustering  stars,  contiguous 
to  each  other,  like  the  collections  that  construct  our  milky-way.  The  other,  on  the 
contrary,  being  real  and  possibly  at  no  very  great  distance  from  us.  The  changes  I 
have  observed  in  the  great  milky  nebulosity  of  Orion,  23  years  ago,  and  which  have 
also  been  noticed  by  other  astronomers,  cannot  permit  us  to  look  upon  this  phenomenon 
as  arising  from  immensely  distant  regions  of  fixed  stars.  Even  HUYGHENS,  the  dis- 
coverer of  it,  was  already  of  opinion  that,  in  viewing  it,  we  saw,  as  it  were,  through 
an  opening  into  a  region  of  light.  (See  Systema  Saturnium,  pages  8  and  9.)  Much 
more  would  he  be  convinced  now,  when  changes  in  its  shape  and  lustre  have  been 
seen,  that  its  light  is  not  like  that  of  the  milky-way,  composed  of  stars.  To  attempt 
a  guess  at  what  this  light  may  be,  would  be  presumptuous. 

"If  it  should  be  surmised,  for  instance,  that  this  nebulosity  is  of  the  nature  of  the 
zodiacal  light,  we  should  then  be  obliged  to  admit  the  existence  of  an  effect  without 
its  cause.  An  idea  of  its  phosphorical  condition,  is  not  more  philosophical,  unless  we 
could  show  from  what  source  of  phosphorical  matter  such  immeasurable  tracts  of 
luminous  phenomena  could  draw  their  existence  and  permanency ;  for,  though  minute 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  31 

changes  have  beeri  observed,  yet  a  general  resemblance,  allowing  for  the  difference  of 
telescopes,  is  still  to  be  perceived  in  the  great  nebulosity  of  Orion,  even  since  the  time 
of  its  first  discovery." 

Sir  WILLIAM  HERSCHEL,  in  his  "  Astronomical  Observations  relating  to  the  Con- 
struction of  the  Heavens,"  in  the  Philosophical  Transactions  for  1811,  has  a  detailed 
discussion  of  his  observations  on  the  nebula  of  Orion,  which  I  quote  almost  in  full : 

"  In  the  year  1774,  the  4th  of  March,  I  observed  the  nebulous  star  which  is  the 
43d  of  the  Connaissance  des  Temps  [G.  P.  B.,  734],  and  is  not  many  minutes  north  of 
the  great  nebula ;  but  at  the  same  time  I  also  took  notice  of  two  similar,  but  much 
smaller,  nebulous  stars :  one  on  each  side  of  the  large  one  and  at  nearly  equal  distances 
from  it  [G.  P.  B.,  543  and  848].  Fig.  37  [of  the  Plates  to  the  Phil.  Trans.,  1811, 
Fig.  1 1  of  this  work],  is  a  cop}'-  of  a  drawing  which  was  made  at  the  time  of  observation. 
In  1783  I  examined  the  nebulous  star  [734],  and  found  it  to  be  faintly  surrounded 
with  a  circular  glory  of  whitish  nebulosity,  faintly  joining  to  the  great  nebula.  About 
the  latter  end  of  the  same  year  I  remarked  that  it  was  not  equally  surrounded,  but 
most  nebulous  towards  the  south.  In  1 784  I  began  to  entertain  an  opinion  that  the 
star  was  not  connected  with  the  nebulosity  of  the  great  nebula  of  Orion,  but  was  one 
of  those  which  are  scattered  over  that  part  of  the  heavens.  In  1801,  1806,  and  1810 
this  opinion  was  fully  confirmed  by  the  gradual  change  which  happened  in  the  great 
nebula,  to  which  the  nebulosity  surrounding  this  star  belongs.  For  the  intensity  of 
the  light  about  this  nebulous  star  had  by  this  time  been  considerably  reduced  by  the 
attenuation  or  dissipation  of  the  nebulous  matter ;  and  it  seemed  now  to  be  pretty 
evident  that  the  star  is  far  behind  the  nebulous  matter,  and  that,  consequently,  its  light 
in  passing  through  it  is  scattered  and  deflected  so  as  to  produce  the  appearance  of  a 
nebulous  star.  A  similar  phenomenon  may  be  seen  whenever  a  planet  or  a  star  of  the 
ist  or  2d  magnitude  happens  to  be  involved  in  haziness;  for  a  diffused  circular  light 
will  then  be  seen,  to  which,  but  in  a  much  inferior  degree,  that  which  surrounds  this 
nebulous  star  bears  a  great  resemblance. 

"When  I  reviewed  this  interesting  object  in  December,  1810,  I  directed  my 
attention  particularly  to  the  two  small  nebulous  stars  by  the  sides  of  the  large  one, 
and  found  that  they  were  perfectly  free  from  every  nebulous  appearance ;  which  con- 
firmed not  only  my  former  surmise  of  the  great  attenuation  of  the  nebulosity,  but 
also  proved  that  their  former  nebulous  appearance  had  been  entirely  the  effect  of  the 
passage  of  their  feeble  light  through  the  nebulous  matter  spread  out  before  them.  The 
i  Qth  of  January,  1 8 1 1 , 1  had  another  critical  examination  of  the  same  object  in  a  very 
clear  view  through  the  4O-feet  telescope ;  but  notwithstanding  the  superior  light  of 
this  instrument,  I  could  not  perceive  any  remains  of  nebulosity  about  the  two  small 
stars  which  were  perfectly  clear,  and  in  the  same  situation,  where,  about  thirty-seven 
years  before,  I  had  seen  them  involved  in  nebulosity. 

u  If  then  the  light  of  these  three  stars  is  thus  proved  to  have  undergone  a  visible 
modification  in  its  passage  through  the  nebulous  matter,  it  follows  that  its  situation 
among  the  stars  is  less  distant  from  us  than  the  largest  of  the  three,  which  I  suppose 
to  be  of  the  8th  or  9th  magnitude.  *  *  *  *  From  the  very  considerable  changes 
which  I  have  observed  in  the  arrangement  of  its  nebulosity,  as  well  as  from  its  great 


32  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

extent,  this  inference  seems  to  have  the  support  of  observation ;  for  in  very  distant 
objects  we  cannot  so  easily  perceive  changes  as  in  near  ones,  on  account  of  the  smaller 
angles  which  both  the  object  and  its  changes  subtend  at  the  eye.  The  following 
memorandum  was  made  when  I  viewed  it  in  1774:  'Its  shape  is  not  like  that  which 
Dr.  SMITH  has  delineated  in  his  Optics,  although  somewhat  resembling  it,  being  nearly 
as  in  Fig.  37;  from  this  we  may  infer  that  there  are  undoubtedly  changes  among 
the  regions  of  the  fixed  stars ;  and,  perhaps,  from  a  careful  observation  of  this  lucid 
spot,  something  may  be  concluded  concerning  the  nature  of  it.'  In  January,  1 783, 
the  nebulous  appea'rance  differed  much  from  what  it  was  in  1 780,  and  in  September 
it  had  again  undergone  a  change  in  its  shape  since  January. 

"March  13,  181 1.  With  a  view  to  ascertain  such  obvious  alterations  in  the  dis- 
position of  the  nebulous  matter  as  may  be  depended  on,  I  selected  a  telescope  that, 
had  the  same  light  and  power,  which  thirty-seven  years  ago  I  used  when  I  made  the 
above  mentioned  drawing ;  and  the  relative  situation  of  the  stars  remaining  as  before, 
I  found  that  the  arrangement  of  the  nebulo&ity  differs  considerably.  The  northern 
branch  N  still  remains  nearly  parallel  to  the  direction  of  the  stars  a  6  [543,  734]  ; 
but  the  southern  branch  S  is  no  longer  extended  towards  the  star  d  [905] ;  its  direction 
is  now  towards  the  star  e  [843],  which  is  very  faintly  involved  in  it.  The  figure 
of  the  branch  is  also  different ;  the  nebulosity  in  the  parallel  P  F  of  the  three  stars 
being  more  advanced  towards  the  following  side  than  it  was  formerly." 

DISCUSSION    OF    THE    VARIATIONS    IN    FORM,    ETC.,    ACCORDING    TO    HERSCHEL. 

In  his  memoir  of  1811,  Sir  WILLIAM  HERSCHEL  recites  three  ways  in  which  the 
nebula  of  Orion  has  changed  between  1774  and  181 1.  These  are: 

(a)  The  stars  543  and  848  which  were  nebulous  in  1774,  as  seen  through  a  5^- 
foot  reflector,  were  afterwards,  1 8 1 1 ,  found  to  be  free  from  nebulosity  with  a  telescope 
of  the  same  power,  and  no  nebulosity  could  be  detected  about  them  even  with  the  40- 
foot  telescope. 

(6)  The  Messierian  branch  which,  in  the  drawing  of  1774,  was  directed  towards 
star  905,  in  181 1  was  directed  toward  the  star  843,  and 

(c)  The  figure  of  the  lucid  part  had  altered ;  in  particular  the  nebulosity  in  the 
direction  of  the  parallel  having  moved  further  towards  the  east. 

These  points  could  scarcely  be  intelligently  considered  as  long  as  we  possessed 
only  the  very.rough  drawing  of  1774,  Fig.  1 1,  as  a  record  of  HERSCHEL'S  observations. 
The  conclusions  of  HERSCHEL,  which  are  never  to  be  lightly  doubted,  were,  in  this 
case,  not  easily  examined,  since  no  knowledge  was  at  hand  as  to  the  instrumental 
means,  the  manner  of  observation,  or  the  amount  of  time  spent  upon  the  subject. 

The  preceding  copy  of  the  observations  in  detail  made  by  HERSCHEL  upon  the 
nebula,  enables  us  to  examine  with  more  care  the  data  upon  which  these  conclusions 
rest.  It  must  not  be  supposed,  however,  that  all  the  evidence  considered  by  HERS  JHEL 
is  there  reduced  to  writing,  since  it  is  plain,  from  the  memoir  of  181 1,  that  these  were 
the  merest  notes  which  served  to  remind  him  of  former  work  done.  We  have  from 
these  notes,  however,  a  much  more  certain  clue  to  the  data  really  available  to  HER- 
SCHEL himself,  and  we  may  with  more  confidence,  though  still  with  circumspection, 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  33 

examine  his  conclusions.  It  must  not  be  forgotten  that  Sir  JOHN  HERSCHEL  has  pur- 
posely omitted  all  the  figures  which  accompany  the  memoir  ( 1 8 1 1 )  of  his  illustrious 
father  from  his  "List  of  figured  Nebulae,"  in  the  Introduction  to  his  General  Catalogue 
of  Nebulae,  as  he  says  "they  do  not  profess  to  be  resemblances."  It  is  quite  true  that 
so  much  as  this  is  stated  by  Sir  WILLIAM  HERSCHEL,  on  page  272  of  this  memoir,  but  it 
is  evident  that  we  must  partially  except  from  this  rule  Fig.  37  of  the  nebula  of  Orionj 
for  Sir  WILLIAM  HERSCHEL  expressly  says  that  the  nebula  was,  in  1774,  "nearly  as  in 
Fig.  37,"  and  he  proceeds  to  predicate  a  change  of  shape  on  evidence  derived  from 
this  drawing. 

The  figure  of  this  drawing  is  like  a  Greek  £1,  and  I  have  been  able  to  reproduce 
its  general  shape  by  viewing  the  nebula  (1875)  through  the  small  finder  of  the  2 
6-inch  refractor  of  the  Naval  Observatory  (2^  aperture,  magnifying  power  about  15 
diameters),  and  M.  TROUVELOT  has  made  a  sketch  of  this  appearance,  which  roughly 
agrees  with  Fig  n.  As  is  stated  by  SECCHI  in  his  memoir  of  1868,  it  is  the  very 
dark,  straight  channel  between  nebula  Mairanni  and  the  main  nebula  that  determines 
the  shape  of  some  of  the  older  drawings  to  be  similar  to  that  of  a  Greek  omega  with 
an  elongated  base. 

In  HERSCHEL'S  figure  the  north  base  of  the  omega  is  not  sufficiently  extended 
toward  the  west  to  agree  with  the  present  appearance. 


FIG.  12.     KEELER,  1878. 

The  accompanying  Fig.  12  (for  comparison  with  HERSCHEL'S)  was  drawn  by  Mr. 
J.  E.  KEELER,  instructor  in  physics  at  the  Johns  Hopkins  University  of  Baltimore, 
January  23,  1878,  with  a  telescope  having  an  aperture  of  2.5  inches;  power,  75. 

We  may  now  consider  the  points  (a),  (6),  (c)  in  detail.     With  regard  to  the  first, 
which  concerns  the  (formerly)  nebulous  stars  543  and  848,  the  MS.  journal  of  obser- 
vations enables  us  to  see  that,  although  the  nebula  was  observed  33  times        til,  11 
APP.  V 5 


34  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

no  case  is  the  nebulosity  about  these  stars  explicitly  mentioned  as  observed,  although 
its  absence  is  remarked.  Neither  MAIRAN  (1733),  LONG  (1742),  LE  GENTIL  (1758), 
MESSIER  (1771),  LEFEBVRE  (1779),  nor  SCHROETER  (1794),  mention  this  nebulosity, 
and  however  little  negative  evidence  should  in  general  avail,  it  would  seem  that  here 
was  sufficient  to  establish  a  strong  probability,  if  not  a  certainty,  that  the  observation 
of  1774  with  the  weak  5^-foot  telescope  was  erroneous  in  ascribing  a  nebulous 
envelope  to  these  two  stars.  It  is  to  be  noted  that  several  of  the  above  named 
authorities  specially  examined  MAIRAN'S  nebula  (about  the  star  734),  and  the  nebulos- 
ities about  543  and  848  could  hardly  have  escaped  them  all.  Moreover,  the  drawing 
bears  the  marks  of  being  mainly  a  memorandum,  as  it  is  not  possible  to  certainly  iden- 
tify all  the  stars  without  his  description.  The  5^ -foot  reflector  was  probably  the  first 
telescope  that  HERSCHEL  himself  made  (in  the  winter  of  i773~'74),  and  before  this 
time  he  had  not  had  extensive  opportunities  to  familiarize  himself  with  the  appearance 
of  nebulae,  so  that,  as  STRUVE  justly  remarks  (Obs.  sur  la  nebuleuse  d'Orion,  page  97) 
this  observation  of  1774  is  not  of  the  same  weight,  as  those  made  somewhat  later.* 
The  showing  of  nebulous  halos  to  bright  stars  is  a  not  uncommon  defect  of  reflectors, 
and  it  seems  quite  possible,  and  in  the  light  of  contemporary  work  even  probable,  that 
some  such  accidental  error  is  here  recorded. 

With  regard  to  (a),  then  it  seems  probable,  from  a  consideration  of  all  the 
evidence,  that  no  material  change  took  place  in  the  appearance  of  these  stars  from 
1733  (MAIRAN)  to  1794  (SCHROETER),  and  in  1811  HERSCHEL  describes  them  to  be  as 
they  now  are. 

(b).  Direction  of  the  Messierian  branch. — In  1771  MESSIER  figures  this  branch 
directed  to  (and  including)  the  star  843,  as  does  also  SCHROETER  in  1 794.  HERSCHEL 
describes  it  in  1774  as  directed  to  905,  and  as  later  he  found  it  to  be  directed  to  843, 
a  change  was  supposed  to  have  taken  place.  To  admit  HERSCHEL'S  explanation  would 
be  to  suppose  the  branch  to  have  been  in  1771  at  the  star  843,  to  have  moved  to  905 
(distant  about  4^0  by  1774  and  back  again  to  843  by  1794.  This  is  manifestly  im- 
probable, and  the  error  arose  in  the  defective  drawing  of  1774,  which,  as  before,  served 
only  to  mislead. 

(c).  With  regard  to  the  motion  of  the  whole  nebula  toward  the  east,  or  rather 
that  part  of  it  in  the  parallel  of  0J ',  it  is  plain  that  the  idea  was  first  suggested  to  HER- 
SCHEL by  comparing  his  drawing  of  1774  with  the  nebula  as  he  saw  it  later  in  the 
heavens. 

It  may  be  at  once  declared  that  no  such  motion  has  taken  place  as  his  drawing, 
compared  with  modern  observations,  would  indicate;  but  in  order  to  test  the  question 
of  any  motion  in  this  direction,  I  have  excerpted  from  the  various  observations  and 
drawings  such  details  as  bear  on  this  and  allied  points,  and  they  follow  immediately 
after  this  paragraph. 

*  L'argument  principal  de  W.  HERSCHEL  en  faveur  des  changements,  consiste  dans  son  observation  que  la  ne"bu- 
Iosit6  qui  entoure  ]'e"toile  de  MAIRAN,  lui  a  paru  autrement  disposed  et  beaucoup  plus  faible  en  1810  qu'en  1774,  et 
que  deux  petites  e"toiles  situ^es  dans  le  voisinage  de  Pe"toile  de  MAIRAN  avaient  en  1810  entierement  perdu  la  nelnilositd 
qu'il  avait  remarque"e  auteur  d'elles  en  1 774.  fivideinment  de  pareilles  observations  peuvent  facilement  avoir  leur  origiue 
dans  quelque  de~faut  optique  de  1'instrnment  ou  en  d'autres  deceptions ;  mais  avec  un  observateur  aussi  consommd  et 
eirconspect  que  l'e"tait  W.  HERSCHEL  m6me  cette  remarque  perdrait  tout  sou  poids,  s'il  n'y  avait  encore  a  conside"rer 
qu'en  1774  il  ne  posse"dait  encore  ni  des  instruments  assez  parfaits,  ni  1' experience  qu'il  a  gagne"e  plus  tard. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  35 

SITUATION    OF    THE    TRAPEZIUM   WITH    REGARD    TO    THE   ADJACENT   NEBULOSITY,    ETC. 

1656.  HUYGHENS.  The  trapezium  involved  in  nebula;  the  south  #  of  Sinus  magnus  is 
filled  with  nebulosity.  The  north  shore  of  Sinus  magnus  much  as  at  present. 
685,  708,  741  in  bright  nebulosity. 

1673.  PICARD.  The  trapezium  much  as  in  HUYGHENS  as  to  following  nebulosity.  628 
and  619  are,  however,  in  a  dark  space  (V)  surrounding  them  and  extending 
toward  the  W.  and  S.  The  south  y2  of  Sinus  magnus  filled  with  fainter  nebu- 
losity :  its  north  shore  in  the  same  general  direction  as  at  present.  685,  708, 
741  in  fainter  nebula. 

1 742.  LONG.  The  trapezium  involved  in  nebulosity.  The  south  part  of  Sinus  magnus 
filled  with  nebulosity.  The  north  shore  much  as  at  present.  685,  708,  741  in 
bright  nebulosity :  724  free  from  nebulosity.  The  parallel  of  &  is  involved 
east  of  708,  but  not  so  far  as  741. 

1758.  LE  GENTIL.  The  trapezium  in  a  dark  space  (V).  The  western  end  of  Sinus 
magnus  filled  with  nebulosity ;  its  southern  y2  also  nebulous.  The  line  of  its 
northern  shore  somewhat  as  a  very  small  telescope  shows  it  to-day ;  i.  e.,  with 
the  s.  f.  corner  of  ff  cut  off.  685,  708,  741  are  in  darkness.  The  extreme 
northern  star  of  the  drawing  is  probably  479.  The  parallel  of  &  is  involved 
east  of  708. 

1771.  MESSIER.  The  trapezium  involved,  the  following  star  being  just  on  the  preced- 
ing edge  of  Sinus  magnus.  No  darker  space  about  trapezium.  The  parallel 
through  9'  and  following  is  involved  in  nebulosity  from  &  to  beyond  741  ;  i.  e., 
the  space  called  Sinus  magnus  at  present  has,  according  to  MESSIER,  its  south 
part  filled  with  nebulosity.  The  north  shore  is  much  as  small  telescopes  show 
it  to-day,  the  s.  f.  part  of  6  being  cut  off.  685,  708,  741  on  the  southern  edge 
of  the  bright  nebulosity,  but  in  the  darker  part.  The  Messierian  branch  goes 
to  843. 

1 774.  HERSCHEL  (from  the  drawing).  The  trapezium  is  involved,  but  its  two  follow- 
ing stars  are  on  the  edge  of  the  Sinus.  The  north  shore  of  the  Sinus  is  on  the 
parallel  of  6 1 9  and  640.  The  general  shape  of  this  gulf  is  much  as  it  is  to-day, 
but  its  situation  is  entirely  different.  685,  708,  741  involved  in  bright  neb- 
ulosity. 

1776,  Nov.  ii.  HERSCHEL  (MS.).  "The  greatest  glare  is  about"  the  trapezium. 
685,  708,  741  "almost  free  from  any  glare."  The  Sinus  magnus  was  totally 
dark. 

1778,  Jan.  25  HERSCHEL  (MS.).  "At  the  eastern  side  the  rays  seem  to  form  an 
equilateral  triangle"  with  685,  708.  ?  At  74 1  "it  bends  towards  the  east  at  an 
angle  of  1 10°  to  1 20°",  much  as  in  MESSIER.  The  north  shore  of  Sinus  magnus 
is  concave  towards  the  south,  different  from  MESSIER.  The  angle  between  the 
south  and  east  shores  of  G  is  about  75°  to  70°.  In  MESSIER  it  is  about  90°. 

1783,  Nov.  3.  HERSCHEL  (MS.).  "Just  close  to  the  'trapezium'  it  is  totally  black 
for  the  short  space  of  a  few  seconds"  (V).  In  the  Sinus  magnus  "a  small 
distinct  nebula  of  an  extended  shape."  (See  observation  of  181 1,  Jan.  19.) 


36  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1806,  Feb.  ii.     HEESCHEL  (MS.).     Trapezium  completely  involved.     685,  708,  741 
completely  free. 

1810,  Dec.  31.     HERSCHEL  (MS.).     724  in   "very  faint  nebulosity."      "The  black 
space  near  the  four  stars"  (Sinus  magnus)  "is  much  contracted." 

1811,  Jan.  19.     HERSCHEL  (MS.).     "  2  of  the  4  stars  (of  the  trapezium)  are  within  the 

nebulosity.''     The  Sinus  magnus  "  is  much  contracted." 

I  include  the  results  of  LEFEBVRE  (1779)  and  SCHROETER  (1797-^78)  in  this  com- 
parison. 
1779.     LEFEBVRE.     The  trapezium  completely  in  the  Sinus  magnus.     685,  708,  741 

free  from  nebulosity. 

1 794.  SCHROETER.  SCHROETER  made  his  drawing  of  1 794  on  a  chart  in  which  MES- 
SIER'S  positions  of  the  stars  were  taken  as  a  basis,  and  this  at  the  outset  will 
account  for  a  considerable  amount  of  distortion  in  the  drawing.  Thus  SCHROE- 
TER'S  drawing  gives  G.  P.  B.  523  about  50"  west  of  479,  while  it  is  in  fact  east 
of  479  by  over  150".  Hence,  in  a  part  of  the  nebula  quite  close  to  0',  we 
may  expect  misplacement  of  the  various  minor  features  by  as  much  as  3'  in 
some  cases.  But  when  the  features  are  important,  and  still  more  when  two  are 
to  be  compared  which  SCHROETER  saw  in  the  same  field  of  view,  we  may  rely 
with  more  confidence  upon  the  drawing  as  it  stands,  and  in  nearly  every  case, 
though  often  only  after  much  pains,  I  have  succeeded  in  satisfying  myself  as 
to  the  portion  intended  to  be  represented.*  From  the  drawing  and  text  the  fol- 
lowing may  be  established : 

The  Messierian  branch  passes  through  843  ;  the  three  following  stars  of  the  trape- 
zium are  in  a  dark  space  (V).     685,  708,  741  are  free  from  nebulosity. 

The  parallel  through  &  and  following  it  is  entirely  immersed  in  nebulosity ;  that 
is,  the  south  %  of  what  is  now  the  Sinus  magnus  is  nebulous. 

The  angle  of  685-741,  and  the  north  short)  of  the  Sinus,  is  70°. 

From  all  of  this  I  conclude  that  HERSCHEL'S  point  (c)  is  not  established. 

OBSERVATIONS  OF  LEFEBVRE  (1779). 

ROZIEK'S  Observations  sur  la  Physique,  volume  xxii  (page  34  and  Plate  I,  Fig.  3), 
for  1 783,  contains  a  letter  to  the  Abbd  ROZIER  from  M.  LEFEBVRE,  which  I  extract  in 
full. 

"OBSERVATIONS  SUR  LES  NEBULEUSES  D'ORION;  par  M.  LEFEBVRE,  Pretre  a 
VOratoire,  Professeur  de  Physique  du  College  de  Lyon. 

"A  1'occasion  de  la  ressemblance  que  M.  de  MAIRAN  soup9onne  entre  la  matiere  de 
a  lumiere  zodiacale,  celle  de  Paurore  boreale  et  celle  de  ces  n^bulosites  qui  accom- 
pagnent  quelques  e'toiles,  j'ai  eu  la  curiosite'  de  revoir  la  ndbuleuse  de  Pepee  d*  Orion 
dont  la  figure  m'avoit  toujours  paru  diffdrente  de  celle  qui  lui  ont  donnde  MM. 
HUYGHENS  et  de  MAIRAN.  La  voici,  telle  que  j'ai  cru  Pappercevoir  le  15  fevrier  1779, 
par  un  ciel  tres-pur,  a  onze  heures  et  demie  du  soir,  ayant  a-peu-pres  1 7  degrds  de 
hauteur.  Voyez  Plane.  I,  fig.  3  [our  fig.  13]. 

"  Les  sept  e'toiles  qui  la  composent  m'ont  paru  entitlement  hors  du  nuage,  sur- 

*  If  it  were  worth  while  a  corrected  drawing  could  have  been  made,  as  in  the  case  of  HUYGHENS,  etc. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


37 


tout  les  trois  inferieures  qui  larssoient  entr'elles  et  le  image  un  intervalle  obscur  bien 
tranchd,  a  1'exception  de  celle  qui  en  est  la  plus  proche,  qui  peut  me  laisser  quelque  doute. 
"M.  de  MAIRAN  croyait  deja  que,  depuis  1756  [misprint  for  1 1656]  (temps  de 
1'observation  de  M.  HUYGHENS)  cette  ndbulosit^  avoit  eprouvc*  quelque  changement 
II  seroit  aujourd'hui  bien  plus  considerable,  et  surtout  la  position  des  e*toiles, 
de'tache'es  du  nuage,  sembleroit  indiquer  qu'au  moins  la  n^bulositd  n'est  produite  par 
aucune  de  ces  Voiles.  Voyez  les  fig.  de  la  PI  I.  La  fig.  ire  est  celle  de  M.  Huyghens, 
en  1656.  La  fig.  2,  celle  de  M.  Mairan  en  1725.  (Ces  deux  figures  sont  renversees.)  La 
fig.  3  est  droite,  et  represents  la  nebulosity  telle  qu'elle  a  paru  cette  annee  1779,  observee 
avec  un  telescope  de  y/2  pieds." 

This  figure  is  but  eight 
years  later  in  date  than 
MESSIER'S.  and  by  no 
means  so  detailed.  The 
principal  differences  and 
resemblances  are  as 
follows : 

(a)  Both  have  the  Mes- 
sierian  branch;  in  LEFEB- 
VRE  the  distance  from  d 
Orionis  to  the  end  of  this 
branch  is  5  times  the  dis- 
tance apart  of  stars  685 
and  741  of  G.  P.  BOND'S 
Catalogue  [128"],  while, 
according  to  MESSIER, 


the   latter   distance   is   6 
times  the  former ; 

(by  the  stars  685,  708, 
and  741  are  detached 
from  the  nebula  in  LE- 
FEBVRE'S  drawing,  on  the 
edge  of  it  in  MESSIER'S  ; 
the  outline  of  LEFEBVRE'S 
drawing  would  nearly  fol- 
low a  line  of  equal  light 
on  MESSIER'S,  except  in 
the  Messierian  branch; 

(c)  the  four  stars  of  the 
trapezium  are  quite  out- 
side of  all  nebulosity  in 
LEFEBVRE,  quite  inside 
according  to  MESSIER. 

The  angle  of  the  opening  ot  the  "jaws"  is  almost  the  same  as  given  by  LE  GENTIL, 

namely,  75°  in  LEFEBVRE  to  56°  (LE  GENTIL). 


FIG.  13.     LEFEBVRE,  1779. 


38  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

It  is  also  noteworthy  that  LEPEBVRE  uses  the  same  conventional  sign  to  represent 
a  nebulosity  gradually  shading  off  to  nothing,  as  does  Pi  CARD  (see  fig.  4),  namely, 
the  ragged  and  saw-shaped  boundary.  This  drawing  is  hardly  more  detailed  than 
HUYGHENS',  although  nearer  the  present  shape  of  the  nebula,  as  seen  with  very  small 
telescopes. 

OBSEKVATIONS  OF  SCHKOETEE  (i794-'98). 

In  BODE'S  Jahrbuch,  1798,  p.  198,  SCHROETER  has  a  few  observations  on  the 
nebula  in  Orion,  from  which  I  quote.  These  observations  are  subsequently  given  in 
full,  but  the  following  notes  will  serve  to  settle  one  or  two  doubtful  points  in  the  sub- 
sequent accounts : 

In  "the  central  parts  of  MESSIER'S  drawing"  no  less  than  18  new  stars  were  seen 
(Jan.  7,  1 794),  "  und  atisser  diesen  iui  Nebel  nahe  nord-  und  westlich  bey  den  ein 
Trapezium  bildenden  4  Sternen,  Zwey  kleine  hellere  Nebelflecken,  in  deren  Mitte 
sich  zwischendurch,  ein  hellerer  doch  sehr  matten  Nebelpunct  zeigte.  Auch  haben 
Zwey  gedachter  18  Sterne,  gleich  dem  Huygenischen  Nejbelsterne,  ihren  eigenen  vom 
iibrigen  getrennten  Nebel  um  sich." 

These  "Zwey"  may  be  i  and  a  of  his  drawing  of  1 794  [Fig.  14],  but  of  these  only 
a  answers  to  the  description,  and  as  no  mention  is  subsequently  made  of  a  change  it 
is  quite  likely  that  for  north  and  west  we  should  read  south  and  east  when  c  and  b  would 
be  referred  to. 

I  know  of  no  other  explanation.    He  also  makes  the  following  notes  : 

(1)  685,  708,  741  are  in  a  dark  space,  different  from  MESSIER,  685  being  just  on 
the  edge  of  the  Frons. 

(2)  The  Sinus  maynus  is  darker  than  the  surrounding  background  of  the  heavens. 

(3)  The  Proboscis  minor  is  described. 

In  the  Aphroditographische  Fragments  of  SCHROETER  (p.  243  and  Plate  II)  may  be 
found  a  resume  of  his  work  upon  the  Orion-nebula,  from  which  I  quote  largely,  partly 
because  this  work  is  now  difficult  to  obtain,*  and  partly  because  of  its  intrinsic  value. 
As  OTTO  VON  STRUVE  has  justly  remarked  in  his  memoir  on  the  same  nebula,  no  doubt 
can  be  entertained  of  the  good  faith  of  SCHROETER,  nor  of  his  general  acuteness  arid 
accuracy  as  an  observer.  The  apparent  discredit  into  which  his  works  have  fallen 
seems  to  be  largely  due  to  the  unwillingness  of  astronomers  to  follow  him  in  his  the- 
oretical conclusions,  and  partly  also  to  the  diffuse  form  in  which  he  gives  them.f 

"BEMERKUNGEN  UBER  ORIONS  LICHTNEBEL. 

"  Verschiedene  Augen  mit  verschiedenen  Fernrohren  bewaffnet,  diirften  zwar 
diesen  merkwiirdigen  Lichtnebel  unter  verschiedenen  Witterungs-und  sonstigen 

*  I  am  indebted  to  the  courtesy  of  President  ELIOT,  of  Harvard  University,  and  to  the  kindness  of  the  authorities 
of  Harvard  College  Observatory,  for  the  use  of  the  copy  from  which  I  quote. 

t  We  find  in  Mem.  R.  A.  S.,  vol.  iii,  p.  188,  a  foot-note  to  a  paper  of  Sir  JOHN  HERSCHEL'S,  in  which  his  opinion 
of  SCHROETER'S  drawing  of  nebula  Orionis  is  given,  which  we  quote:  "I  have  been  guilty,  I  find,  of  a  piece  of  invol- 
untary injustice  to  M.  SCHROETER  in  omitting  to  mention  him  among  the  observers  of  this  nebula.  I  am  indebted  to 
my  esteemed  and  admired  correspondents,  Dr.  OLBERS  and  M.  HARDING,  for  a  reference  to  his  Aphrodttographisclie 
Fragmente,  for  his  observations  on  this  nebula;  and  the  title  of  the  work  will  plead  my  excuse  for  the  omission. 
The  representations  of  it  there  given  are,  however,  so  dreadfully  bad  as  almost  to  convert  the  excuse  into  a  justi 
fication  *  *  *  *" 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION,  39 

Nebenumstanden  immer  etwas  verschieden  sehen ;  bis  jetzt  sind  wir  aber  in  der 
Schopfungskunde  zu  weit  zuruek,  als  dass  es  niitzlich  sein  sollte,  alle  Bruchstiicke 
von  Beobachtungen  solcher  Art  der  Nachwelt  zu  umstandlichern  Vergleichungen, 
Prufungen  und  Folgerungen  aufzubewahren.  In  solcher  Hinsicht  konnen  daher  auch 
folgende  Bemerkungen  niitzlich  werden,  die  mit  zwei  vorzuglich  lichtstarken  Instru- 
menten,  iiamlich  einem  ^fiissigen  besonders  aber  mit  dem  27fiissigen  Reflector 
angestellt  sind. 

^  Bei  der  da  von  verfertigten  Tab.  II  befindlichen  Charte,  die  in  der  Gestalt  und 
Begranzung  des  Lichtnebels  von  den  bisherigen  bekannten  Zeichnungen  merklich 

abweichet,  habe 
ich  die  in  des 
Herrn  Prof.  BODE 
Vorstellung  der 
Gestirne  Tab. 
XXX,  fig.  6,  mit 
enthaltene  Zeich- 
nung  nach  den 
Beobacht  u  n  g  e  n 
des  Herrn  MES- 
SIER, in  dem  Un- 
terschiede  der 
Aufsteigung  und 
Abweichung, 
jedoch  nach  ei- 
nem g  r  6  s  s  er  n 
Maass-stabe  zum 
Grundegelegt,  so 
dass  diejenigen 
altern,  b  is  h  e  r 
darin  bekanntge- 
wesenen  Sterne, 
bei  welch  en 
nichts  zu  bemer- 
ken  vorgefallen, 
o  h  n  e  Bezeich- 
nung  geblieben, 
die  hinzugekom- 
menen  aber  mit 
Buchstaben  be- 


FlG.  14.      SCHROETER,    1794. 


zeichnet  nachgetragen  worden  sind.  Eine  wirkliche  mikrometrische  Messung  fand  ich 
theils  wegen  der  Feinheit  und  Dunkelheit  der  Gegensttinde  unsicher  und  fast  unmog- 
lich,  theils  aber  auch  fur  meinen  Zweck  uberfliissig,  well  em  im  Schiitzen  geiibtes 
Augenmaass  in  solchen  Fallen  oft  sicherer  ist.  Urn  aber  desto  leichter  die  dunklern 
Sterne  von  den  hellern  zu  unterscheiden  und  in  der  Folge  wieder  zu  finden,  ist  die 


4O  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Characteristik  der  abnehmenden  Lichtstarke,  wenn  sie  auch  gleich  aus  bekannten 
Griinden  nur  sehr  beylaufig  und  einem  veranderlichen  Wechsel  ausgesetzt  sein  diirfte, 
unter  der  Charte  nach  mehrern  und  wenigern  Sternstrahlen  ausgezeiget. 

"  Vornehmlich  habe  ich  mich  beflissen,  die  ausserst  verschwachenen  Granzen  und 
die  Gestalt  des  hellern  und  schwachern  Lichtnebels,  die  ich  auch  mit  kleinern  Tele- 
scopen  und  Vergrosserungen  controliret  und  eben  so  gefunden  haben,  moglichst 
genau  zu  bestimmen. 

"Nach  diesen  vorlaufigen  Bermerkungen  sind  die  kleinen  Sterne,  welche  ich  bis 
jetzt  ausser  den  von  dem  beriihmten  Astronomen  Herrn  MESSIER  ausgezeichneten 
nach  und  nach  gefunden  habe,  folgende : 

"In  a  und  b  [D  and  A?]  zeigen  sich  im  27fiissigen  Reflector  zwei  sehr  kleine 
Kernpunctgen,  die  beide  gleich  dem  Huygenischen  Nebelsterne  in  einen  besondern  vom 
iibrigen  unterschiedenen  Nebel  gehiillt  sind,  besonders  erkannte  ich  in  a  [D  !]  den 
hellern  Nebelpunct  den  6ten  Janner  1 794,  und  ein  ahnlicher  schien  westlich  bey  /3 
durchzublicken  [following  part  of  C  ?]  :  wenigstens  war  hier  der  Lichtnebel  merklich 
heller,  und  es  schien  sich  in  diesen  Puncten  die  Kraft  des  27fiissiges  Telescops  der 
Auflosbarkeit  des  Nebels  zu  nahern.  Einen  ahnlichen  vom  iibrigen  getrennten  Nebel 
haben  die  beiden  Sterne  C  [Not  in  MESSIER;  our  E !]  und  d  [570]  den  ich  besonders 
deutlich  den  7ten  Janner  1794,  ab  nach  10  U.  mit  250  mal  Vergrosserung  des  27- 
fiiss.  Telescops  erkannte,  und  von  welchen  des  erstern  Nebel  sich  bios  nordlich  mit 
dem  iibrigen  Nebel  zu  vermischen  schien.  *****  Weiter  fand  ich  nordlich 
zunachst  unter  9±  einen  entfernten  dunkeln  Stern  i  [635]  bei  dem  sich  westlich  ein 
etwas  dunkler  Strich  im  Nebel  zeigte  [W  =:  lacus  secchii]  *  *  *  *  *  q  [650  +  653  ?] 
liegt  in  einem  langlichen  dunkeln  Striche,  welcher  den  ostlichen  Lichtnebel  vom 
iibrigen  trennt  und  ist  auch  im  I3fiiss.  Ren*,  sichtbar.  *****  Der  Anblick  des 
Lichtnebels  selbst  ist  damit  prachtvoll  und  erscheint  in  mehrere  von  einander  getrennte 
Theile  aufgeloset,  die  sich  durch  Zeichnung  nicht  ausdriicken,  sondern  nur  sehen 
lassen,  und  zwiscben  welchen  man  die  dunklere  Himmelsluft  unterscheidet.  So  wie 
er  unter  der  lichtstarken  183  mal  Vergross.  des  27fiiss  Reflectors  ins  Auge  fallt,  ist  er 
Zunachst  bei  0^  und  zwar  von  yd  bis  nordlich  unter  k  und  q  hin  am  hellesten. 

Von  a  [D?]  bis  gegen  den  Stern  £  [843]  sprosst  von  ihm  ein  langer  Zweig  gegen 
Siiden  ab,  welcher  auch  mit  schwachern  Fernrohren  sichtbar  ist.  Viel  schwacher  ist 
hingegen  ein  gegen  Osten  nach  dem  Stern  /*  [848]  hin  absprossender  Lichtstrief,  der 
mit  dem  I3fiiss.  Reflector  nur  zum  Theil  erkannt  wird. 

Er  fallt  gleich  dem  hellern  siidlichen  Streife  gegen  seine  Spitze  hin  immer  matter 
und  zuletzt  so  ausserst  matt  ab,  dass  seine  Endspitze  mit  dem  27fiissigen  Telescope, 
dessen  grosser  Lichtstarke  ungeachtet,  nicht  vollig  genau  bestimmt  werden  kann. 
Reizend  ist  sein  Anblick  der  Vorstellungskraft  des  Naturforschers :  denn  hochst  wahr- 
scheinlich  hat  er  gleich  dem  siidlichen  Lichtstreife  gegen  unser  Auge  eine  schrage 
Lage  und  erstreckt  sich  in  einem  unermesslich  entfernten  Himmelsraume  bis  zu  einer 
Entfernung  fort,  die  sich  der  Fasslichkeit  des  Beobachters  entziehet.  P^ine  almliche 
schrage  Lage  scheinet  auch  von  dl  an  bis  zur  Endspitze  des  lichtern  Nebels  rj  hin 
statt  zu  finden :  denn  auch  heir  wird  ein  gleicher  matterer  Abfall  des  Lichts  merklich, 
und  der  Nebel  fallt  von  a  [D!]  nach  ;/  hin  eben  so  matt  und  unbegranzt,  als  nach  <? 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  41 

[843]  und  //  [848]  bin  ab.  Der  westliche  Nebel  hingegen  ist  bei  z,  x,  n,  o,  p,  und 
ft  an  sich  sehr  schwach  jedoch  von  A,  x,  V  an  bis  zur  westlichen  Granze  wieder  etwas 
heller. 

********** 

Bemerkenswerth  ist  iibrigens  noch  (I)  dass  01  sammt  die  beiden  ostlichen  folgen- 
den  Sternen,  bei  alien  diesen  Boebachtungen  mit  mancherlei  starken  und  schwachern 
Fernrohren  gesehen,  nicht  innerhalb,  sondern  ausserhalb  des  Nebels  im  dunkeln  Eaume 
liegt,  so  dass  der  Lichtnebel  nur  durch  den  ostlichsten  streichet,  da  doch  die  altern 
Zeichnungen  diese  Sterne  innerhalb  des  Nebels  setzen. 

Unentschieden  bleibt  es  freilich,  ob  der  verdienstvolle  MAIRAN,  welcher  damals 
schon  urtheilte,  dass  Orions  lichter  Nebel  seit  HUYGHENS  Zeiten  einige  Veranderung 
erlitten  zu  haben  scheine  [S.  des  Herrn  DE  LA  LANDE  Astronomie  §  837],  dieser  Meinung 
ungeachtet  nicht  sorgfaltig  genug  in  der  Zeichnung  seiner  Gestalt  verfahren,  oder  auch 
zu  schwache  Fernrohre  dabei  angewandt  habe :  allein  seine  Zeichnung,  welches  die 
beiden  hellesten  solche  3  Sterne  innerhalb  des  Lichtnebels  setzt,  weichet  von  der  des 
Herrn  MESSIER  zu  auffallend  und  selbst  diese,  welche  gedachte  drei  Sterne  ebenfalls 
innerhalb  des  lichten  Nebels  enthalt,  von  der  meinigen  wieder,  wenn  gleich  nicht 
auffallend,  doch  so  viel  ab,  dass  die  unverganglichen  Namen  eines  MAIRAN  und  MES- 
SIER die  Zukunft  zu  einer  desto  sorgfaltigern  Vergleichung  der  bis  herigen  und  kiinftgen 
Beobachtungen  auffordern.  Zumal  da  auch  meine  mit  7-  und  4fiissigen  Telescopen 
und  einem  lofussigen  DOLLOND  geschehenen  Vergleichungen  mit  dieser  Bemerkung 
iibereinstimmen.  Immer  wird  indess  eine  solche  Vergleichung  mit  vieler  Behutsamkeit 
bios  auf  das  Wesentlichste  zu  richten  sein,  weil  bei  der  verwachsenen  Unbegranztheit 
dieses  Lichtnebels  wohl  eben  nicht  zu  erwarten  ist,  dass  selbst  gleichzeitige,  mit 
gleichen  Instrumenten  versehene  Beobachter  seine  Gestalt  durchgehends  piinctlichst 
gleich  entwerfen  wurden." 

SCHROETER,  in  a  letter  to  BODE,  dated  December  10,  1797,  published  in  BODE'S 
Jahrbuch  for  1801,  p.  126,  in  referring  to  his  observations  of  the  nebula  of  Orion  of 
1797,  says:  "Eine  gewiss  merkwiirdige  Beobachtung  ist,  dass  ich  in  Theilen  fixer 
Nebel,  namlich  des  Nebels  im  Orion  *  *  *  wirklich  zufallige  Veranderungen 
wahrgenommen  habe."  The  observations  to  which  he  refers,  follow  here. 

In  the  iii  volume  of  SCHROETER'S  "  Beytrage  zu  den  Neuesten  Astronomische 
Entdeckungen,"  p.  149,  there  is  found  a  long  account  of  his  "Beobachtungen 
iiber  zufallige  Veranderungen  fixer  Lichtnebel,"  from  which  I  give  the  following 
synopsis  setting  forth  his  observations  nearly  in  full,  but  in  some  cases  abbreviating  the 
account  of  conclusions  reached.* 

After  a  preliminary  reference  to  the  known  variation  in  the  light  of  some  of  the 
fixed  stars,  SCHROETER  says:  "For  several  years  I  believe  I  have  made  out  similar 
variations  in  the  remarkable  nebula  of  Orion,  and  these  variations  I  have  seen  not 
alone  in  its  contained  stars,  but  also  in  the  nebulous  matter  itself."  He  remarks  that 
deception  in  such  things  may  easily  arise,  and  that  in  order  to  be  sure  of  variation  it  is 

*  In  the  copy  of  this  work  belonging  to  the  Naval  Observatory  (ist  ed.,  Gottiugen,  1800),  the  paging  is  wrong  after 
page  1 60  to  the  end  of  the  book.  The  page  after  page  160  is  numbered  149,  so  that  the  pages  149-160  occur  twice. 
The  references  are  to  the  pages  as  printed, 

APP.  y — 6 


42  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION 

necessary  to  confine  the  attention  to  some  of  the  smaller  and  well  known  parts — the 
parts  being  taken  so  small  that  they  are  equally  bright  all  over — and  to  compare 
such  parts  not  only  among  themselves,  but  also  to  the  light  of  the  neighboring  small 

fixed  stars,  in  order 
to  discriminate  the 
truth  from  delusion 
by  frequent  com- 
parisons under  all 
circumstances  of 
observation  and  all 
atmospheric  con- 

FlG.  15.      SCHROETER,   1797. 

ditions.  The  re- 
sult of  such  an  examination  is  given  in  the  figure  in  Aphroditographische  Fragmente  (our 
Fig.  14)  from  the  comparison  of  which  with  earlier  figures,  SCHROETER  came  to  the 
conclusion  that  some  change  had  taken  place.  In  reference  to  such  conclusions 
further  observations  were  made,  which  are  given  in  great  detail.  The  italics  are 
SCHKOETER'S  own. 

January  25,  1797.  The  dark  space  (Sinus  magnus)  appeared  to  SCHROETER 
"  uncommonly  black,  and  darker  than  ever  before,"  and  "und  es  fiel  mir  als  eine  ganz 
neue,  fur  die  Folge  wahrscheinlich  sehr  instructive  Bemerkung  hochst  merkwiirdig 
auf,  dass  mir  dieses  Mai  sofort  mit  dem  ersten  Blicke  in  diesem  schwarzdunkeln  eingreifen- 
den  Raume,  ein  neuer  heller,  aber  dusserst  matter  Lichtstreifen  ins  Gesicht  fiel.  Er  gieng 
nach  Fig.  i  (our  Fig.  1 5 )  von  a.  Ms  fi  sudlich  sum  Osten  quer  durch  solchen  ganzen  dunkeln 
Maum,  und  westlich  von  s  nach  <?  schieh  ihm  parallel,  ebenfalls  etwas  Helles  hinzu- 
streifen."  This  observation  was  confirmed  by  HARDING,  who  saw  it  somewhat  brighter 
than  SCHROETER,  who  describes  it  as  very  faint,  and  seen  only  at  intervals.  "  Grleich 
merkwiirdig  war  es,  dass  wir  beide  ubereinstimmend  in  dem  deutlichen  ostlichen  Licht- 
streifen a,  /?,  und  zwar  in  7,  6,  zwey  dusserst  matte,  aber  doch  merklich  hellere  Lichtpunct- 
chen  fanden,  die  uns  als  ausserst  entfernte  kleine  dunkle  Sternchen  ins  Auge  fielen, 
und  von  welchen  y  \j£o  of  Index-Chart]  das  augenfalligste  war.  Aber  auch  das  war 
nicht  alles :  denn  eben  so  merkwiirdig  und  lehrreich  war  es  mir,  das  ich  etwas  nord- 
licher,  einen  zweyten  neuen  eben  so  matten  Lichtstreifen  rj  entdeckte,  [o  TT  of  the  Index- 
Chart!?]  welcher  ostlich  in  solchen  dunkeln  Eaum  strich,  auch  1  und  i  zwei  feme,  matte 
Lichtpiinctchen  fand,  wovon  I  das  augenfalligere,  mein  in  der  den  Aphroditographi- 
schen  Fragmenten  angehiingten  Charte  mit  I  bezeichnetes  ist  [781?]. 

Wer  es  bedenkt,  mit  welcher  sorgfalt  ich  nicht  nur  Orions  Nebel  nach  seinen 
kleinern  Theilen  iiberhaupt,  sondern  auch  besonders  den  in  ihn  eingreifenden,  mir 
immer  vorzuglich  merkwiirdig  gewesenen,  so  ausgezeichnet  schwarzdunkeln  Raum,  Jahre 
hindurch,  sowohl  mit  dem  27  als  i3fussigen  Reflector,  bey  der  heitersten  Luft 
gemustert  hatte,  der  wird  es  selbst  fiihlen,  dass  ich  gedachte  drei  Lichtstreifen  sammt 
den  Lichtpuncten  7,  6,  und  i,  mit  allem  Grunde  fur  ganz  neue  JErscheinungen  halten 
musste,  weil  ich  bey  alien  jenen  altern  Beobachtungen,  da  ich  zum  Theil  viel  feinere 
weniger  augenfallige  Theile  von  Orions  Nebel  bemerkte,  und  seinen  hineintretenden 
schwarzdunkeln  Raum  naher  zu  erforschen  suchte,  von  diesen  Erscheinungen  nicht 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  43 

die  geringste  Spur  wahrgenommen  hatte,  die  mir  nun  aufeinmal  und  zwar  sdmmtlich  in  sol- 
cliem  kunkeln  Eaume  zum  Theil  mit  der  erster  Blicke  ins  Gesicht  fielen" 

********* 

1797,  Feb.  17.  The  atmospheric  and  other  conditions  being  as  good  as  on  the 
25th  of  January,  the  nebula  was  not  seen  as  on  that  date,  but  different  in  the  following 
respects:  both  the  streaks  of  faint  light  £<?  and  a/3  were  seen,  the  first  much  better 
than  on  January  25,  but  in  the  eastern  one,  a/3,  only  the  brighter  northern" point  y 
[£o]  was  seen,  while  <S  was  invisible.  Whatever  the  condition  of  the  atmosphere,  as 
SCHROETER  justly  remarks,  either  the  western  streak  £<?  had  increased  in  light,  or  the 
small  nucleus  d  had  diminished.  Furthermore,  the  Sinus  magnus  had  encroached  upon 
the  nebulous  portions  so  that  the  distance  from  the  Trapezium  was  no  more  than  the 
distance  between  its  two  southern  stars.  In  the  spot  x  (SCHROETER'S  Fig.  i)  the  space 
was  perfectly  black.  As  noted  in  his  large  chart  (see  our  Fig.  14),  it  is  quite  different 
from  the  appearance  in  his  Fig.  i  (our  Fig.  1 5). 

For  several  years  SCHROETER  had  seen  the  three  bright  stars  south  following  dl 
(BOND'S  685,  708,  741)  in  a  completely  dark  space,  according  to  his  own  account,  and, 
indeed,  he  founded  an  argument  for  a  change  between  HUYGHENS'  time  and  his  own 
on  this  very  circumstance.  The  former  appearance  and  the  one  now  observed  he 
describes  thus:  "der  Lichtnebel  dicht  an  dem  ostlichen  dieser  drey  Sterne  [685,  708,  741] 
hinstrich.  Jetzt  hingegen  stand  der  Lichtnebel  von  solchem  ostlichen  Stern  *  *  *  nacli 
Fig.  2  betrdchtlich  ab.  Dagegen  trat  aber  von  dem  Lichten  urn,  Q-^  befindlichen  hellern  Licht- 
nebel ab,  ein  etwas  matterer  aber  doch  sehr  deutlicher  Lichtnebel  von  a  Fig.  2  Us  an  62. 
(See  Fig.  16).  Abermahls  ein  Umstand,  den  ich  mit  aller  Gewissheit  nicht  so  gefunden 
hatte,  weil  ich  deisen  Umstand  vor  dem  Druck  meiner  Charte  nochmals,  und  zwar 
unter  andern  auch  mit  dem  lofiissigen  DOLLOND  nachsahe,  und  von  dem  mattern 
Lichtnebel  a,  gleich  als  vorhin,  iiberall  nichts  fand."  Such  repeated  revisions  do  not 
allow  us  to  think  of  a  deception  in  this  matter,  and  in  this  detail  again  SCHROETER 
thinks  there  can  be  no  doubt  of  a  change. 

1 797,  Dec.  27.  On  this  date  SCHROETER  observed  in  the  Sinus  magnus  "eine  blasse 
dusserst  matte  Lichtschicht  wieder  an  eben  derselben  Stelle,  wo  wir  vorhin  die  LichtschicM 
a/3,  Fig.  i,  beobachtet  hatten,  jetzt  gieng  sie  aber  nicht  ganz,  sondern  nur  bis  auf  */3  durch 
denfinstern  Eaume  namlich  von  a  bis  gegen  7,  Fig.  i  (our  Fig.  15)  und  von  den  beyden 
Lichtpuncten  y,  d,  fand  ich  uberall  nichts  wieder"     At  the  same  time  the  Sinus  magnus 
was  not  so  black  as  common,  but  notably  brighter.     The  state  of  the  atmosphere  was  so 
good  that  SCHROETER  cannot  ascribe  this  change  to  its  influence. 

1 798,  Jan.  25.    In  bright  moonlight,  the  Sinus  magnus  was  again  found  to  be  black 
as  compared  to  the  surrounding  sky,  but  no  trace  was  seen  of  aft  and  e  <?.     In  the 
place  of  the  streak  a/3,  which  at  the  last  ob- 
servation   had  extended  jA  of  the  distance 

from  the  edges  of  the  Sinus  nothing  was 
seen,  while  the  nucleus  y  had  reappeared, 
and  in  spite  of  the  bright  moonlight,  was  as 
bridit  as  the  first  of  the  five  outer  satel- 
lites of  Saturn  (Tethys).  This  remarkable  FlG' l6'  ScHROETER' 


44  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

observation  was  confirmed  on  the  29th  of  January,  when,  in  spite  of  the  moon,  which 
was  not  far  from]  the  nebula  and  ^  full,  this  point  was  again  seen :  "  von  dem  in  eben 
solchen  dunkeln  Raume  befindlichen,  iveit  Tiellern  Fixsterne  1  Fig.  i  (Fig1.  15)  liingegen 
fand  ich  so  wie  von  i  und  von  den  bey  den  Lichtstreifen  uberall  kerne  Spur.  Offeribar  hatte 
also  der  Punct  y  neues  und  zwar  viel  stdrkeres  Licht,  als  vorJiin  erhalten." 

1 798,  March  2.  *  *  *  "  denn  jetzt  sahe  ich  so  gar  bey  vollem  hellen  Mondlichte 
nicht  nur  solchen  intermittirend  durchblinkenden  Lichtpunct,  sondern  auch  wieder  etwas 
Streifiges  vom  Lichtstreifen  aft  Fig.  i."  This  streak  SCHROETER  maintains  must  have 
an  entirely  new  one  since  none  had  been  seen  there  on  several  previous  occasions,  and 
since  it  was  seen  so  plainly  in  full  moonlight.  He  has  represented  this  in  his  Fig.  3 
(our  Fig.  1 6)  and,  as  we  see,  it  only  extends  to  the  nucleus  y.  On  this  occasion,  too,  the 
Sinus  magnus  was  three  or  four  times  as  dark  as  the  surrounding  sky.  Near  x  (Fig.  1 6) 
the  entire  space,  which  had  previously  been  seen  intermixed  with  light,  was  now 
wholly  black.  1798,  March  13  [misprinted  1788],  not  the  least  trace  was  found  of 
aft  and  e <?,  although  on  March  2  a/3  had  been  seen  as  far  as  y.  Of  y  itself  SCHROETER 
saw  no  certain  sign,  although  both  he  and  HARDING  had  glimpses  of  one  or  two  brighter 
spots  Jn  the  dark  space  of  the  Sinus  magnus.  SCHROETER  gives  a  proof  of  the  goodness  of 
the  atmosphere  in  that  the  small  star/  [707]  of  his  large  chart  was  plainly  seen.  This 
is  about  1 1.2  magnitude.  "  Without  any  doubt  this  streak  aft  had  in  eleven  days  lost 
the  greater  part  of  its  light/'  On  the  following  evening,  March  14,  "bey  ausseror- 
dentlich  reiner  Luft"  absolutely  nothing  was  seen  of  the  two  streaks  of  light,  and  no 
certain  trace  of  7,  although  it  was  suspected. 

Again,  the  three  stars  [BOND  685,  708,  741],  were  now  all  in  a  completely  dark 
space,  while,  according  to  SCHROETER'S  large  chart  the  nebulosity  passed  through  741, 
the  easternmost  of  these  three,  and  on  February  17,  1797,  the  western  one  [685]  was 
likewise  involved.  On  this  date  also  a  new  appearance  was  observed  "  at  the  first 
glance,"  which  is  shown  in  our  Fig.  16.  The  projection  rj  was  in  about  the  same  posi- 
tion as  the  projection  77  of  Fig.  1 5,  but  in  an  entirely  different  direction  and  of  a  different 
magnitude. 

1798,  March  19.  The  star  /  south  following  the  trapezium  was  seen,  and  also  the 
small  round  nebula  a  [D  of  the  Index-Chart].  (See  SCHROETER'S  large  chart,  our 
Fig.  14).  The  atmosphere  was  good,  but  no  trace  was  seen  of  the  dark  space  q,  which 
formerly  had  been  so  plain.  *  *  *  *  "  Genug  der  dunklere  in  meiner  Charte  mit 
q  \lacus  Lassettii]  bezeichnete  Zwischenstrich,  den  ich  vor  etlichen  Jahren  so  deutlich  gesehen 
hatte,  war  verschwunden  und  an  seiner  Stelle  Lichtnebel  entstanden" 

The  Messierian  branch  or  proboscis  major  had  been  correctly  figured  by  SCHROETER 
in  his  large  chart,  but  the  proboscis  minor  was  only  well  seen  with  his  2  7-foot  reflector, 
the  13-foot  being  hardly  adequate.  This  was  in  1793  and  1794.  Five  years  later, 
1798,  March  19,  the  proboscis  minor  was  seen  so  bright  with  the  1 3-foot  reflector  that 
it  was  for  a  time  supposed  to  be  veritably  the  Messierian  branch;  this  latter  was  so 
faint  as  to  make  deception  easy,  much  fainter  than  the  former.  If  this  had  been  so  in 
MESSIER'S  time  (1771)  it  would  not  have  been  seen;  from  y  (of  SCHROETER'S  large 
chart,  our  Fig.  14)  onwards,  only  a  faint  trace  of  it  was  seen. 

1798,  Dec.  10.  Under  very  favorable  conditions  y  and  d  of  (Fig.  15)  were  seen 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


45 


as  one  "  sehr  matten  schwachen  und  kaum  erkennbaren  Lichtpunct,"  while  no  trace 
of  £<?  was  seen. 

1799,  Dec.  10.  The  state  of  the  sky  was  particularly  fine,  and  there  was  seen  a 
faint  trace  of  the  streak  aft  extending  from  north  to  south  half  of  the  distance  across 
the  Sims  magnus.  The  nucleus  y  was  again  seen,  not  as  formerly,  but  as  "  eine  ver- 
waschene  gedrdngtere  Helligkeit,  die  einem  verwaschenen  Kerne  eines  kleinen  entfernten 
Cometen  sehr  ahnlich  war."  This  would  be  the  appearance  £o  in  a  telescope  like 
SCHROETER'S  at  the  present  time.  Nothing  was  seen  of  8  nor  of  «<?,  and  the  Sinus 
east  of  a/3  was  extraordinarily  black,  more  so  than  it  had  before  been  seen. 

In  the  Nachtrag  to  the  Zweite  Abtheilung  of 
SCHROETER'S  work  (Beytrage,  p.  222)  reference  is 
made  to  his  figure  39  [our  Fig.  17],  and  descrip- 
tions of  observations  given,  of  which  a  summary 
follows. 

a  of  his  large  chart  [Fig.  14]  was  first  discov- 
ered in  1793,  and  the  central  nucleus  of  a  1794, 
Jan.  6.  His  figure  39  shows  plainly  that  a  is 
our  D,  and  the  central  nucleus  is  probably  647 
and  651  seen  as  one  star.  This  object  had  always 
been  "em  sehr  feiner,  schwer  zu  unterscheidener 
Gegenstand."  On  Feb.  2,  1800,  while  examining 
"other"  parts  of  the  nebula,  SCHROETER  was 
astonished  to  see  "dass  mir  dieser  kleine,  schwer 
zu  erkennende  riindliche  Nebel,  von  selbst  in 
ausserordentlich  starken  Lichtglanze  ins  Gesicht 
fiel,"  much  brighter  than  ever  before.  It  was  at 

least  three  times  as  bright  as  the  brightest  parts  of  the  nebula  about  it.  On  Feb.  5  the 
same  appearance  was  observed,  and  a  was  again  estimated  to  be  three  times  brighter 
than  the  brightest  parts  about  it.  On  Feb.  1 1  it  had  lost  its  brightness  and  become  as 
before.  On  Feb.  12,  under  good  circumstances,  the  observation  of  Feb.  u  was  con- 
firmed. The  "  spherical"  mass  a  was  compared  with  the  dimensions  of  the  trapezium, 
and  its  diameter  was  ^  of  the  distance  between  the  two  brightest  stars ;  that  is,  its  diam- 
eter was  in  the  neighborhood  of  10". 

On  Feb.  2 1  a  was  hardly  so  bright  as  the  surrounding  parts  of  the  nebula.  * 
On  pages  231  et  seq.,  is  found  a  description  of  what  must  be  the  lacus  Lassellii,  though 
the  scale  of  the  figure  is  grossly  wrong.  It  contains  a  star,  SCHROETER'S  q,  which  may 
be  663,  or,  as  I  at  first  supposed,  650  +  653.  This  dark  stripe  was  first  seen  in  1 795. 
In  1797-98-^99  it  was  not  seen  In  Feb.,  1800,  the  part  of  o  of  the  Index-Chart 
containing  lacus  Lassellii  and  all  east  of  it  was  invisible.  On  Feb.  21  and  23  the 
distance  of  the  eastern  limits  of  G  from  635  (SCHROETER'S  y\  measured  on  the  line  635 
to  734,  was  only  %  of  the  distance  635-734,  while  formerly  it  had  been  #. 

On  G.  P.  BOND'S  engraving  of  1865  the  distance  from  635  to  734  is  21  parts,  and 
the  distance  from  635  to  the  brighter  following  edge  of  a  is  6  of  the  same  parts,  or  a 
little  more  than  tf,  so  that  the  last  observations  of  SCHROETER  represent  the  present 


46 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


appearances  most  nearly.  If  on  BOND'S  drawing  we  lay  off  ^  of  the  distance  635-734 
on  this  line,  the  point  so  fixed  falls  within  the  nebula  Mairanni,  and  thus  a  suspicion  is 
created  that  the  channel  a  q  of  Fig.  1 7  was  at  first  used  by  SCHROETER  to  represent  the 
dark  space  following  0,  and  latterly  used  as  lacus  Lassellii. 

I  think,  however,  that  there  can  be  no  doubt  but  that  the  small  drawings  after 
1 794  refer  to  the  lacus  Lassellii,  and  to  the  parts  near  the  trapezium,  and  I  believe  the 
following  list  of  identifications  to  be  in  the  main  correct.  A  reference  to  the  original 
manuscript  drawings  of  SCHROETER  would  be  required  to  satisfy  all  doubts.  This  I 
have  not  been  able  to  make. 


SCHROETER'S 
Number. 

G.  P.  BOND'S 
Number. 

Remarks. 

SCHROETER'S 
Number. 

G.  P.  BOND'S 
Number. 

Remarks. 

Star. 

467 

Not  numbered. 

Star. 

734 

Star. 

497 

Not  numbered. 

/ 

78i 

o 

427??   . 

y« 

848 

P 

435?? 

7 

784 

Star. 

449 

Not  numbered. 

J 

822 

505 

h 

793? 

n 

49° 

c 

843 

Star. 

479 

y 

823 

Star. 

554 

z 

855 

Star. 

55i 

a 

881 

Star. 

506 

Star. 

905 

u 

524?? 

V 

889 

m 

558?? 

b 

567?  575? 

Bright  part  of  A?  cl 

d 

570 

c 

573??? 

Nucleus  in  E?? 

k 

580 

Dark  space 

Lacus  Secchii 

i 

635 

1 

635 

9 

650-653? 

Star. 

669 

Not  numbered. 

r 

700 

a 

647?? 

Bright  part  of  D? 

In  the  preceding  extracts  I  have  omitted  reference  to  several  circumstances  con- 
nected with  the  nebulous  star  discovered  by  HUYGHENS  north  following  the  trapezium, 
since  this  star  is  outside  the  limits  which  I  have  proposed  to  examine,  and  since  the 
evidence  adduced  is  not  so  striking  as  -what  has  been  given.  I  have  also  omitted  many 
of  the  conclusions  drawn  by  SCHROETER  from  his  observations,  but  have  endeavored 
scrupulously  to  give  the  essence  of  the  observations  themselves,  as  well  as  references 
to  the  state  of  the  atmosphere,  a  most  important  datum.  It  is  impossible  to  read  the 
account  of  these  observations  without  giving  to  them  a  certain  faith  and  credence,  and 
it  must  be  remembered  that  in  most  of  these  observations  SCHROETER  had  the  assist- 
ance of  HARDING,  which  is  an  additional  argument  for  their  acceptance. 

From  a  consideration  of  all  his  own  and  some  previous  observations,  the  conclu- 
sion finally  reached  by  SCHROETER  is  as  follows,  (Beytrage,  p.  172):  "Fixe  Lichtnebel 
sind  zum  Theil  eben  so  gut  einem  bald  stdrkern,  bald  schwachern  Zuftusse  oder  Modification 
des  Lichts,  einem  wahren  zufalligen  Lichtwechsel  unterworfen,  als  es  gedrdngtere  LicJit- 
pliasen  der  Fixsterne  oder  Sonnen  sind" 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  47 

This  was  his  own  conclusion  from  the  sum  total  of  his  six  years'  (1794-1800) 
observations.  I  find  it  impossible  to  follow  him  in  all  details  which  made  up  his  judg- 
ment, as  I  believe  that  the  grossly  erroneous  drawing  of  1 794,  distorted  by  the  errors 
of  MESSIER'S  star  positions,  was  accountable  for  some  of  the  greatest  variations. 

The  following  seem  to  me  the  points  established  by  his  series  of  drawings  and 
observations : 

Drawing  of  1794.     (FiG.  14.) 

The  southern  apex  of  the  Huyghenian  region  (E)  was  prolonged  to  the  south 
farther  than  it  now  is.  This  is  represented  in  the  same  manner  by  MESSIER  (i  771)  by 
the  drawing  of  1 794  and  by  Fig.  1 7.  The  three  independent  drawings  (for  SCHROETER 
tells  us  that  after  1 794  he  made  another  large  chart)  agree  in  giving  a  bright  curved 
southern  horn  to  E,  which  certainly  does  not  exist  at  present. 

His  nucleus  c  must  be  near  E  [or  602],  and  was  bright  in  1794-1800. 

His  nucleus  b  must  be  the  bright  mass  A  or  part  of  c,  or  possibly  the  star  575. 

The  lacus  Secchii  is  laid  down. 

i  must  be  635,  and  the  star  following  it  669.  From  the  relation  of  a  to  these  it 
follows  that  a  is  either  D  or  the  star  647. 

The  dark  channel  containing  q  has  its  south  end  where  lacus  Lassellii  ought  to  be. 
The  relations  of  his  stars  k  [580],  r  [700],  and  734  prove  that  on  the  drawing  of  1794 
q  is  650.  In  the  later  sketches  [Figs.  15,  16,  17]  this  is  in  doubt,  and  I  believe  that 
lacus  Lassellii  may  have  been  correctly  located  at  its  south  end,  and  the  existence  of  a 
dark  channel  about  q  established,  and  on  the  final  chart  the  two  dark  spaces  connected. 
I  am  aware  that  this  conclusion  is  doubtful ;  but  after  more  study  than  the  subject 
perhaps  deserves,  it  seems  to  me  correct,  particularly  if  we  remember  that  any  limited 
region  containing  stars  is  likely  to  be  correctly  drawn,  and  that  if  we  can  correctly 
identify  the  stars  the  original  facts  of  observation  may  be  thus  recovered,  provided 
that  the  drawing  is  not  assumed  to  be  without  distortion  over  too  great  an  area. 

Minor  sketches  in  1797-1800.     (Fios.  15,  16,  17.) 

In  the  first  of  these  (1797,  Jan.  25),' Fig.  15,  SCHROETER'S  bridge  is  correctly  laid 
down  (aft).  The  two  nuclei  [7^]  I  do  not  understand.  The  interior  bridge  [in  r 
of  the  Index-Map]  is  also  plainly  laid  down.  The  luminous  space,  x,  is  inexplicable. 

On  Feb.  17,  1797  [Fig.  15],  the  position  of  685  relative  to  the  Frons  is  correctly 
shown.  The  abnormal  appearance  at  the  apex  of  E  is  also  indicated.  In  Fig.  16 
(1798,  Mar.  2),  SCHROETER'S  bridge  is  shown,  as  it  might  easily  be  seen  in  his  reflector, 
and  as  HERSCHEL  (1824)  shows  it. 

In  his  Figs,  i  and  4  (our  Figs.  15  and  16),  the  point  rj  is  inexplicable,  unless  it  be 
the  following  point  of  6  in  the  Index-Map. 

In  his  Fig.  39  (our  Fig.  1 7),  the  nuclei  a,  6,  c  are  shown,  which  I  have  before  iden- 
fied  either  as  D,  A,  E,  or  I,  or  else,  as  is  perhaps  more  probable,  as  647,  575,  and  602?? 

In  this  figure  the  abnormal  shape  of  the  apex  of  E  is  shown,  and  perhaps  lacus 
Lassellii. 


48  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

OBSERVATIONS  OF  BODE  (circa  1800). 

The  following  cut  is  a  reproduction  of  a  drawing-  by  BODE,  given  in  his  Anleituny 
zur  Kenntniss  d.  Gestirnten  Himmels,  p.  166,  and  Plate  i. 


FIG.  18.    BODE,  1800. 
OBSERVATIONS  OF  FLAUGERGUES  (1802). 

In  the  Connaissance  des  Terns  for  i8o2-'o3  (An  XI),  p.  361,-  HONORE  DBS  FLAUGER- 
GUES has  a  note  entitled  "  Observations  de  la  nebuleuse  dj  Orion,"  from  which  the  following 
extracts  are  taken : 

"  J'observe  aussi  depuis  plusieurs  anne'es  cette  nebuleuse  et  j'y  ai  vu  des  change- 
mens  bien  considerables:  un  grand  espace  carre*  d'une  lumiere  faible  qui  e"tait  k  1'occi- 
dent  de  cette  ncibuleuse  ovale,  situde  au  sud  de  cette  n(3buleuse  qui  en  (itait  sdpar^e 
paralt  actuellement  etre  r(3unie  k  cette  derniere  sous  la  forme  d'une  gerbe  lumineuse ; 
enfin  la  partie  inferieure  de  la  n^buleuse  s'est  beaucoup  re'tre'cie  dans  la  partie  occi- 
dentale,  puisque  les  trois  e'toiles  en  droite  ligne  [685,  708,  741]  qui  dans  les  figures 
donndes  par  HUYGHENS  et  MAIRAN,  sont  dans  le  milieu  de  cette  partie,  se  trouvent 
actuellement  tout-a-fait  au  bord  et  meme  souvent  hors  de  la  nebulosite*. 

"  Enfin  je  n'avais  vu  encore  que  trois  e'toiles  dans  le  groupe  marqu^  9  par  HAM- 
STEED  lorsque,  le  18  vend^miaire  au  7  de  matin,  j'en  de'couvris  une  quatrieme  au  sud 
de  ces  trois  e'toiles.  MAIRAN  remarque  qui  suivarit  un  dessin  de  PICARD  du  20  mars, 
1673,  qui  lui  avait  e'te'  communiqu^  par  GODIN  il  y  avait  quatre  e'toiles  dans  ce  groupe. 

"  J'ai  dessine  une  figure  de  cette  ndbuleuse  le  plus  exactement  qu'il  m'a  etc'  pos- 
sible, afin  qu'on  puisse  reconnaitre  les  changemens  qui  pourront  y  arriver  dans  la  suite 
et  je  1'ai  adresse*e  au  Bureau  des  Longitudes  pour  qu'on  puisse  la  consulter  quand 
on  aura  fait  d'autres  observations  analogues." 

This  figure  has  been  sought  for  by  M.  M.  YVON  VILLARCEAN,  MOUCHEZ,  and  ANGOT 
at  the  Bureau  of  Longitudes  and  at  the  Observatory  of  Paris,  but  without  success. 
It  is  probably  now  lost. 

I  have  to  express  my  thanks  to  these  gentlemen  for  the  pains  they  have  taken 
in  this  respect. 

OBSERVATIONS  OF  SIR  JOHN  HERSCHEL  (1824). 
OF    THE    GREAT    NEBULA   IN   ORION.* 

"  Before  proceeding  to  comment  on  former  drawings,  it  will  be  well  to  have  before 
us  a  careful  and  correct  representation  of  its  present  state.  Such  a  one  is  that  in  the 
annexed  drawing,  which  has  been  made  from  a  set  of  drawings  and  notes  taken 

*  Mem.  R.  A.  S.,  vol.  3i,  p.  487. 


FIG.  19.    J.  HERSCHEL,  1826. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  49 

in  several  nights'  observations  in  the  2O-feet  reflector  with  its  full  aperture  in  favorable 
nights,  and  in  the  absence  of  the  moon,  but  principally  on  that  of  Feb.  i,  1824,  and 
compared  afterwards  with  the  real  obiect,  noting  and  correcting  what  stood  in  need  of 

alteration.  The  last  of  these  ob- 
servations was  made  at  Slough 
on  the  3d  of  March,  1826,  with 
the  advantage  of  Mr.  RAMAGE'S 
judgment  as  well  as  my  own, 
when  ail  the  essential  features 
represented  in  the  drawing,  from 
which  this  has  been  finally  and 
very  carefully  copied,  were  dis- 
tinctly seen  by  us  both,  and 
allowed  to  be  truly  depicted. 

"  I  now  come  to  describe  the 
different  parts  of  the  nebula,  in 
order  to  supply  in  some  measure 
the  unavoidable  imperfections  of 
every  drawing,  and  to  notice  the 
discrepancies  between  this  and 
former  accounts. 

"  i.  Trapezium, — These  four  stars  I  shall  designate  by  a,  /?,  yt  6.  They  form  the 
quadruple  star  9  Orionis.  Their  relative  position  is  unaltered  apparently.  Mr.  SOUTH 
has  given  measures  of  their  angles  of  position  and  distance  in  his  paper  (Phil  Trans., 
1826,  part  i). 

"  The  nebula,  which  is  very  bright  in  the  parts  surrounding  the  trapezium,  seems 
(whether  by  the  effect  of  contrast  with  the  dazzling  light  of  these  stars,  or  from  a  real 
deficiency  in  nebulous  matter)  to  have  retreated  from  immediate  contact  with  them,  so 
that  they  appear  in  some  degree  insulated,  and  with  a  darkness  about  them.  This 
would  agree  with  the  idea  of  a  subsidence  of  the  nebula  into  the  stars  by  gravitation. 
But  it  is  probably  only  a  deception.  Mr.  POND  has  made  the  same  remark  of  the 
apparent  insulation  of  the  trapezium,  as  seen  with  Mr.  RAMAGE'S  25-feet  telescope  now 
at  Greenwich. 

"  2.  The  Huyghenian  Region. — The  figure  of  this  portion  is  nearly  a  right-angled 
triangle.  The  forehead  and  occiput  form  exactly  a  rigid  angle,  and  the  confine  between 
bright  light  and  comparative  darkness  on  these  sides  is  extremely  well  defined.  The 
line  of  the  forehead  is  continued  across  the  insertion  of  the  trunk,  offering  an  appear- 
ance as  if  one  well-defined  nebula  were  laid  upon  another,  which  graduates  away 
insensibly  into  what  may  be  called  the  subnebulous  region. 

"  The  Huyghenian  region  is  represented  in  MESSIER'S  engraving  as  of  a  uniform 
brightness  ;  but  this  is  very  far  from  being  the  case,  as  its  illumination  is  extremely 
unequal  and  irregular.  I  know  not  how  to  describe  it  better  than  by  comparing  it  to 
a  curdling  liquid,  or  a  surface  strewed  over  with  flecks  of  wool,  or  to  the  breaking  up 
of  a  mackerel  sky  when  the  clouds  of  which  it  consists  begin  to  assume  a  cirrous 
A  pp.  V- 7 


50  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

appearance.  It  is  not  very  unlike  the  mottling  of  the  sun's  disc,  only  (if  I  may  so 
express  myself)  the  grain  is  much  coarser  and  the  intervals  darker ;  and  the  flocculi, 
instead  of  being  generally  round,  are  drawn  out  into  little  wisps.  They  present,  how- 
ever, no  appearance  of  being  composed  of  small  stars,  and  their  aspect  is  altogether 
different  from  that  of  resolvable  nebulce.  In  the  latter  we  fancy  by  glimpses  that  we 
see  stars,  or  that,  could  we  strain  our  sight  a  little  more,  we  should  see  them.  But 
the  former  suggests  no  idea  of  stars,  but  rather  of  something  quite  distinct  from  them 
I  do  not  find  it  noticed  or  described  in  any  former  account,  but  this  must  be  attributed 
to  the  want  of  light  in  the  telescopes  used,  for  it  is  not  to  be  seen  in  a  seven-foot  New- 
tonian of  six  inches  aperture.  In  MESSIER'S  figure  the  frontal  line  is  very  indistinctly 
marked ;  and  instead  of  preserving  its  direction  all  along,  is  made  to  form  an  obtuse 
angle  or  curve,  following  the  course  of  the  three  stars  e  [G.  P.  B.  685],  <?  [G.  P  B. 
708],  •//  [G.  P.  B.  741],  which  are  there  represented  as  situated  exactly  on  the  edge  of 
the  brightest  part.  This  leads  us  to 

"The  Subnebulous  Region,  in  which  are  situated  the  stars  f,  <?,  77,  A  [G.  P.  B.  724] 
and  ^  [G.  P.  B.  707].  It  is  occupied  by  a  faint  nebulosity,  insensibly  and  very 
gradually  fading  away  into  darkness.  If  we  compare  the  figures  of  HUYGHENS,  PICARI>, 
MESSIER,  and  that  here  presented,  it  would  seem  as  if  the  brighter  portions  of  the 
nebula  had  formerly  extended  over  this  space,  and  were  now  contracting  and  receding 
towards  the  trapezium ;  for  in  the  figures  of  HUYGHENS  and  PICARD  the  three  stars  are 
represented  as  deeply  immersed  within  the  nebula,  and  the  division  which  contains 
them  is  by  far  the  most  considerable  portion  of  the  whole ;  but  as  in  these  there  is  no 
variation  of  shade,  it  is  impossible  now  to  say  where  they  fixed  the  limit  of  what  is  to 
be  considered  as  nebula.  In  MESSIER'S  figure  they  are  placed  precisely  on  the  edge 
of  the  most  luminous  portions,  as  above  mentioned,  while  at  present  they  are  com- 
pletely detached  from  it,  and  the  line  which  they  form  makes  an  angle  of  at  least  45° 
or  50°  with  the  frontal  line.  This  appears  conclusive,  as  the  withdrawing  of  the 
nebula  would  seem  to  have  gone  on  progressively.  Unluckily,  however,  the  observa- 
tions of  LE  GENTIL  will  not  allow  of  this  conclusion.  This  figure,  drawn  on  Marcli 
10,  1758,  and  therefore  sixteen  years  before  MESSIER'S,  represents  it  in  this  respect 
just  as  it  stands  at  present,  the  line  of  the  three  stars  forming  the  same  angle  with  the 
frontal  line ;  and  to  take  away  all  doubt  on  the  subject,  he  says  expressly :  "Les  trois 
e'toiles  que  Messieurs  HUYGHENS  et  PICARD,  et  apres  eux  GODIN  et  DE  FOUCHY,  ont 
vues  en  ligne  droite  dans  ce  que  j'appelle  la  machoire  inferieure,  nous  ont  paru  totale 
ment  de'tache's  de  la  ne'buleuse — elles  e*taient  sur  une  meme  ligne  droite,  et  elles 
faisaient  un  angle  considerable  avec  la  machoire  inferieure."  On  another  night:  "Je 
trouvai,"  he  says,  u*  *  *  que  les  trois  etoiles  qui  dans  les  figures  de  Messieurs 
HUVGHENS  et  PJCARD  sont  dans  la  machoire  inferieure  faisaient  avec  elle  un  angle 
d'environ  40°  ;"  the  meaning  of  which  he  further  explains  by  a  reference  to  letters  in 
his  figure.  The  observation  was  made  with  a  6-feet  Gregorian  telescope,  which  of 
course  would  only  permit  the  brightest  part  to  be  distinguished ;  but  on  turning  on  it 
telescopes  of  8  and  16  feet,  he  assures  us  that  the  three  stars  then  appeared  exactly 
as  HUYGHEKS  and  PICARD  represented  them — i.  e.,  quite  within  the  nebula,  and  of  this 
appearance  he  gives  a  separate  figure. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  51 

1  These  observations  completely  destroy  the  weight  of  any  conclusion  as  to  a 
change,  drawn  from  the  comparison  of  HUYGHENS'  figure  with  MESSIER'S  or  my  own. 
But  how  MESSIER  could  have  overlooked  the  very  remarkable  and  striking  effect  of 
the  frontal  line  and  its  oblique  position  with  respect  to  the  three  stars,  with  LE  GENTIL'S 
figure  and  description  before  him,  and  his  attention  specially  turned  to  the  point,  and 
with  a  telescope  capable  of  showing  the  other  peculiarities  so  well,  is  certainly  sur- 
prising, and  may  lead  to  a  suspicion  that  this  line  has  really  since  become  more 
definite,  and  that  the  nebula  has  retreated.  The  star  ^  [G.  P.  B.  707]  is  not  in  this  figure. 

"Sinus  Gentilii. — The  totally  dark  recess  designated  by  this  name  is  represented  by 
LE  GHNTIL  as  a  very  long,  narrow  exactly  rectangular  cut,  commencing,  not  as  at 
present,  at  a  considerable  distance  to  the  southwest  of  the  star  c  [685],  but  nearly 
opposite  it,  so  that  the  line  joining  <?  [708]  and  e  [685]  produced  actually  enters  the 
Sinus,  and  makes  an  angle  with  its  general  direction  much  less  than  at  present — 
hardly  more  than  15°  or  20°  in  his  figure.  The  angle,  too,  between  the  frontal  and 
occipital  line,  which  at  present  is  just  a  right  angle,  is  represented  by  him  as  very 
obtuse — about  135°.  MESSIER  makes  this  Sinus  dim  and  indistinct,  but  an  approach 
to  the  rectangular  form  of  this  part  (the  vertex)  of  the  nebula  is  perceivable. 

"  Crista  seu  Eegio  Gentiliana. — Represented  by  LE  GENTIL  as  an  exact  well-defined 
rectangle,  whose  length  is  nearly  twice  its  breadth,  and  bounded  on  the  north  follow- 
ing side  by  the  Sinus  which  runs  in  the  direction  of  its  length.  This  figure  presents 
no  resemblance  in  this  part  to  MESSIER'S.  The  rectangle  is  made  to  adhere  to  the 
nebula  by  a  thin  neck  of  less  breadth  than  the  Sinus. 

11  In  MESSIER'S  figure,  a  very  much  greater  extension  is  to  be  remarked  in  the  crest. 
In  his  figure  it  includes  the  star  marked  2,  and  its  boundary  passes  off  to  the  south 
preceding,  a  little  below  the  star  marked  i  (which  stars  he  has  been  at  the  pains  of 
laying  down  by  actual  micrometrical  measure,  and  which  are  inserted  in  my  drawing 
from  his  measures),  and  considerable  strength  and  boldness  is  given  to  its  outline  in 
this  part.  The  portion  next  to  i  is  represented  as  even  brighter  than  the  great  branch 
(Brachium  Messieri).  At  present,  if  nebulosity  exist  in  that  region  (and,  perhaps, 
hardly  any  part  of  the  sky  for  some  degrees  round  can  be  regarded  as  quite  free  from 
it),  it  is  certainly  very  faint ;  and  if  MESSIER'S  figure  is  to  be  trusted,  a  material  altera- 
tion here  must  have  taken  place.  With  respect  to  this  latter  point  he  must  be  allowed 
to  speak  for  himself.  "Le  dessin  de  la  nebuleuse  d'Orion  que  je  presente  a  1'Acadt'mie 
a  e"te  trac^  avec  le  plus  de  soin  qui  m'a  e"te  possible.  La  nebuleuse  y  est  repre'sentee 
telle  que  je  1'ai  vue  plusieurs  fois,  etc."  And  the  engraving  is  inscribed  "Presente'  au 
Roi  le  27  mars  1 774."  It  has  consequently  all  the  authenticity  which  can  be  desired; 
and  the  habit  of  viewing  such  objects  as  its  author  had  acquired  in  his  very  successful 
researches  on  nebulae,  would  seem  to  render  it  little  probable  that  his  eye  would  be 
deceived  in  such  a  point.  Yet,  of  the  two,  it  must  be  confessed  that  this  part  of  LE 
GENTIL'S  approaches  much  the  nearest  to  the  present  appearance,  and  there  is  even  an 
approximation  to  the  rectangular  form  still  perceptible.  This  will  serve  to  show  how 
great  is  the  difficulty  of  representing  such  objects,  and  with  what  caution  evidence  of 
changes  in  them  ought  to  be  received. 

"Eegio  Messieriana. — Brachium  Messieri  seu  Probiscis  Major. — This  arm  was  first 


c2  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

seen,  and  is  very  imperfectly  represented  by  MESSIER;  the  fainter  arm  or  proboscis  has 
escaped  his  observation  altogether,  as  also  has  the  elongated  nebula  between  the  stars 
x  [848]  and  ju  [734],  which  is  of  the  last  degree  of  faintness.  In  favorable  moments 
it  exhibits  a  suspicion  of  a  star  in  its  middle. 

"Mairanni  Nebula  et  Constellatio. — The  curious  form  of  this  nebula  which  throws 
out  a  sort  of  projection  or  tail  just  reaching  to  a  small  star  a,  is  not  depicted  by 
MESSIER,  who  indicates  merely  a  faint  round  nebulosity  encircling  the  star  ^  [734] 
equally  on  all  sides  like  an  atmosphere.  It  consists  of  two  principal  parts :  the  body, 
which  is  the  part  formerly  seen,  and  the  tail,  which  extends  between  the  stars  a  and  e, 
leaving  a  pretty  perceptible  division  or  space  of  fainter  light,  as  if  it  were  about  to 
break  into  two.  The  other  small  stars  ft,  c,  d,  f,  g,  h  are  unaffected  by  nebulosity ; 
z  is  one  of  MESSIER'S  stars  micrometrically  laid  down. 

"Regio  Picardiana. — Only  the  star  v  is  noticed  by  MESSIER.  The  very  curious 
filaments  which  extend  across  or  nearly  across  the  great  Sinus  are  here  noticed  for  the 
first  time.  They  require  distinctness  as  well  as  light.  The  small  island  nebula  minima, 
just  at  the  entrance  of  the  fauces,  appears  as  if  drawing  together  into  a  star.  It 
is  barely  if  quite  detached  from  the  point  of  the  lower  jaw,  which  is  pretty  bright 
and  of  a  ragged  appearance.  The  portion  in  which  the  stars  v  [G.  P.  B.  669],  w 
[G.  P.  B.  663],  y  [G.  P.  B.  652]  are  situated  is  on  the  other  hand  darkish,  and  the 
nebula  appears  as  if  about  to  separate  in  this  place  and  leave  the  extremity  of  the 
lower  jaw  detached.  Between  6  [624]  and  g>  [647],  close  along  the  borders  of  the 
Sinus  at  its  bottom,  is  a  part  much  brighter  than  the  rest,  of  a  small  breadth. 

"Regio  Derhamiana,  etc — From  the  trapezium  there  run  out  branches  or  tails  like 
those  of  comets,  in  the  direction  of  the  star  T  [523]  along  the  confines  of  this  and  the 
Huyghenian  region,  and  across  all  that  space  between  T  [523]  and  o  [479]  and  4;  [449], 
which  lose  themselves  imperceptibly  in  the  very  faint,  diluted  nebulosity  which  fills 
the  Fouchian  region  and  adheres  more  or  less  in  the  form  of  wisps  to  all  the  stars  in 
the  latter,  especially  the  stars  n  [335]  and  p.  The  stars  o  [479]  and  £  [449],  on 
the  contrary,  are  rather  free  of  nebulosity,  being  situated  in  a  darker  portion,  which 
forms  a  natural  separation  between  the  Fouchian  and  Godinian  region.  In  the  latter, 
the  nebula  dies  away  imperceptibly  into  total  darkness." 

HERSCHEL'S  remarks  have  been  quoted  from  largely,  as  his  memoir  served  to  settle 
a  nomenclature  which  has  been  since  adhered  to,  and  as  they  give  a  good  general 
description  of  the  different  parts.  In  his  first  figure  (Fig.  19)  it  is  only  necessary  to 
call  attention  in  passing  to  the  nebulous  filaments  which  he  has  represented  on  the 
north  border  of  Sinus  magnus  near  SCHROETER'S  bridge.  I  conceive  them  to  be,  in 
fact,  representations  of  the  two  bridges  of  SCHROETER  imperfectly  seen. 

OBSERVATIONS  OF  POND.     (1826.) 

The  recession  of  the  nebula  from  the  brighter  stars  is  remarked  upon  by  HERSCHEL 
in  what  immediately  precedes  The  earliest  notice  of  a  recession  of  the  light  of  the 
nebula  from  the  neighborhood  of  the  brighter  stars  in  it  is,  however,  due  to  POND,  who, 
in  1826,  communicated  to  the  Royal  Astronomical  Society  the  results  of  his  observa- 
tions with  RAMAGE'S  reflecting  telescope.* 

*  Mem.  R.  A.  S.,  vol.  2,  p.  93. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


53 


After  describing  the  situation  of  stars  685,  708,  741  of  BOND'S  Catalogue,  he  pro- 
ceeds as  follows  :  "  Now  these  three  stars  are  neither  situated  on  the  edge  of  the  nebula 
as  represented  in  M.  MESSIER'S  plate,  *  *  *  nor  are  they  parallel  to  the  edge;  but 
they  seem  to  be  insulated  from  the  nebula,  the  light  of  which  retires  from  them  in  a 
semicircular  form,  as  if  they  had,  in  some  incomprehensible  way,  either  absorbed  or 
repelled  the  light  from  their  immediate  vicinity.  The  same  appearance  is  observable 
in  the  trapezium,  round  the  four  stars  of  which  the  light  has  also  receded  in  a  very 
analogous  manner,  leaving  them  on  a  comparatively  dark  ground.  In  both  these 
cases  the  impression  on  my  mind  is  that  the  stars  have  been  the  immediate  cause  of 
the  disappearance  of  the  light.  ******  Another  similar  case  is  noticed  a  few 
minutes  distant  from  the  trapezium,  and  the  Astronomer  Royal  concludes  with  an 
expression  of  his  intention  to  communicate  a  drawing  of  this  appearance  to  the  society. 
I  have  not  been  able  to  learn  anything  regarding  this  drawing.  Through  the  kindness 
of  Sir  GEORGE  AIRY  an  unsuccessful  search  has  been  made  for  it  at  Greenwich  and  in 
the  archives  of  the  Royal  Astronomical  Society. 

OBSERVATIONS  OF  LAMONT  (1837). 

LAMONT  speaks  of  his  own  drawing  (Fig.  20)  as  follows  :  "  Meine  Zeichnung  be- 
zieht  sich  nur  auf  den  glanzendsten  Theil  des  Nebels,  der  das  Trapez  umgiebt.  Ver- 
gleicht  man  sie  mit  der  HERSCHEL'  schen 
Darstellung  [in  Mem.  R.  A.  S.,  vol.  2 
(1826)]  so  ergeben  sich  nicht  unbe- 
deutende  Unterschiede  ;  dass  merkwur- 
digste  aber  ist,  dass  der  Refractor  bes- 
timmte  und  begrenzte  Abtheilungen 
erkennen  lasst,  wo  Sir  JOHN  HERSCHEL 
nur  im  Allgemeinen  ein  ungleiches  Licht 
gesehen  hat. 

"Ubrigens  bemerke  ich,  dass  meine 
Absicht  bei  Beobachtung  des  Orion- 
Nebels  dahin  ging,  die  einzelnen  Ab- 
theilungen, so  weit  es  moglich  war, 
micrometrisch  zu  messen,  um  liber  kiinf- 
tige  Aenderung  bestimmt  entscheiden  zu 
konnen  :  dass  aber  auch  eben  desshalb 
in  der  Zeichnung  vorzugsweise  nur  die 
messbareii  Theile  mit  Sorgfalt  darges- 
tellt,  die  schwacheren  und  unmessbaren  Abtheilungen  dagegen  mit  wenigerm  Fleisse 
beriicksichtiget  sind."* 

The  Fig.  20  here  given  of  the  nebula  according  to  LAMONT,  is  copied  from  his 
engraving  published  in  the  work  just  cited.  Through  the  kindness  of  Dr.  DOBERCK, 
astronomer  of  Colonel  COOPER'S  private  observatory  at  Markree  Castle,  I  have  had 


FIG.  20.     LAMONT,  1837. 


Nebelflecken,  Miinchen,  1837,  p.  23.)    It  is  unnecessary  to  go  into  an  examination  of  LAMONT'S  obser- 
vations here,  as  they  have  been  fully  discussed  by  LIAPONOFF  in  his  elaborate  memoir.    We  may,  however,  repeat 
LAMONT'S  remark  that  the  divisions  of  the  Huyghenian  region  (E,  F,  G,  H,  etc.,  of  the  Index-Chart)  are 
down  with  precision. 


54 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


access  to  an  original  pencil  drawing  by  LAMONT'S  own  hand,  dated  February,  1839, 
sent  by  him  to  Markree  Castle  for  comparison  with  a  sketch  by  EDWARD  G.  COOPER? 
esq.  The  engraving  is  a  very  faithful  reproducing  of  the  original,  but  it  is  to  the  latter 
sketch  that  I  have  referred  in  all  comparisons  throughout  this  work,  and  in  general  I 
have  always  referred  to  the  original  engravings  and  not  to  the  wood-cuts  herewith, 
which  are  inserted  principally  for  the  purpose  of  making  text  intelligible. 

OBSERVATIONS  OF  SIB  JOHN  HERSCHEL  (1837). 

The  central  part  of  Sir  JOHN 
HERSCHEL'S  second  and  very 
elaborate  drawing,  made  at  the 
Cape  of  Good  Hope  in  1837,  is 
given  in  Fig.  2 1  From  his  work, 
Astronomical  Observations  at  the 
Cape  of  Good  Hope  (p.  25,  et  seq.), 
I  extract  the  following :  "I  am 
aware  of  but  four  representations 
of  this  nebula  which  have  ap- 
peared since  1824 — one  by  Dr. 
LAMONT,  published  with  his  thesis 
"  Ueber  die  Nebelflecken,"  read 
at  the  anniversary  sitting  of  the 
Bavarian  Academy  of  Sciences, 
August  25,  1837,  and  two  by 
Sig.  RONDONI,  a  Roman  artist. 
The  former,  though  rather  a 
coarsely-executed  figure,  and  con- 
fined solely  to  the  denser  part  of 
the  nebula,  or  those  regions  which 
I  have  termed  the  frons,  occiput, 
and  fauces,  yet  contains  some 
valuable  particulars  respecting  the  apparent  breaking-up  of  the  nebula  (especially 
about  the  frons  and  occiput)  into  patches  and  knots ;  particulars  very  unsatisfactorily 
expressed  in  my  figure  of  1824,  but  in  which  my  observations  of  1834  and  1837  fully 
confirm  Dr.  LAMONT'S  remarks.  In  his  figure  he  has  (perhaps  intentionally)  omitted 
to  express  the  remarkable  effusion  of  the  nebula  from  the  "frons"  and  "proboscis" 
into  what  I  have  termed  the  "Subnebulous  region,"  and  he  has  filled  the  interior  of  the 
trapezium  with  nebula,  a  particular  in  which  we  disagree  decidedly.  The  two  figures 
of  Sig.  RONDONI,  which  are  given  in  the  Report  of  Observations  made  at  the  Collegio 
Romano,  by  the  associated  astronomers  of  the  Gregorian  University,  for  the  years 
1840  and  1841,  are  perhaps  rather  to  be  regarded  as  curious  specimens  of  lithography 
than  as  accurate  representations  of  the  nebula  (such,  at  least,  as  I  have  ever  seen  it), 
which  they  resemble  in  fact  hardly  more  than  they  do  one  another.  *  *  *  *  *  * 


FIG.  21.    J.  HERSCHEL,  1837. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  55 

I  purposely  avoid  all  comment  on  the  remarks  which  accompany  these  two  repre- 
sentations, leaving-  astronomers  to  form  their  own  judgment  on  them.  The  other 
representation  above  alluded  to  is  that  of  Sig.  DEVICO  himself,  in  the  year  1839,  printed 
in  the  Annals  of  the  Collegio  Romano  for  1838,  which,  though  much  less  inaccurate  in 
many  respects  than  Sig.  RONDONI'S,  is  by  no  means  free  from  objection  on  that  score." 

On  pp.  31  et  seq.  of  the  same  work  we  find  Sir  JOHN'S  discussion  "  Of  evidences 
of  change  in  the  nebula." — "  To  the  reader  who  has  never  viewed  this  object  through 
powerful  telescopes,  but  who  is  familiar  with  the  various  representations  which  have 
from  time  to  time  been  made  of  it  (including  my  own  of  1824),  the  number  and  com- 
plexity of  the  various  branches  and  convolutions  now  first  exhibited,  and  the  different 
aspects  under  which  even  the  portions  best  known  are  now  presented,  will  no  doubt 
tend  to  convey  a  strong  impression  of  great  and  rapid  changes  undergone  by  the 
nebula  itself.  I  am  far  from  participating  in  any  such  impression.  Comparing  only  my 
own  drawings  made  at  epochs  (1824  and  1837)  differing  by  thirteen  years,  the  dis- 
agreements, though  confessedly  great,  are  not  more  so  than  I  am  disposed  to  attribute 
to  inexperience  in  such  delineations  (which  are  really  difficult)  at  an  early  period — to 
the  far  greater  care,  pains,  and  time  bestowed  upon  the  later  drawings — and  above  all 
to  the  advantage  of  local  situation  and  the  very  great  superiority  in  respect  both  of 
light  and  defining  power  in  the  telescope  at  the  latter  over  what  it  possessed  at  the 
former  epoch,  the  reasons  of  which  I  have  already  mentioned.  These  circumstances 
render  it  impossible  to  bring  the  figures  into  comparison  except  in  points  which  could 
not  be  influenced  by  such  causes.  Now,  there  is  only  one  such  particular  on  which  I 
am  at  all  inclined  to  insist  as  evidence  of  change,  viz,  in  respect  of  the  situation  and 
form  of  the  "nebula  oblongata"  which  my  figure  of  1824  represents  as  a  tolerably 
regular  oval  extended  very  nearly  in  a  right  line,  or  at  most  but  a  very  little  curved 
upwards  between  the  two  stars  x  =•  No.  120  [G.  P.  B.  781],  and  K  =  No.  136  [G.  P. 
B.  848]  of  the  Catalogue.  Comparing  this  with  its  present  appearance,  as  exhibited 
in  Plate  VIII,  it  seems  hardly  possible  to  avoid  the  conclusion  of  some  sensible  alter- 
ation having  taken  place.  No  observer  now,  I  think,  looking  ever  so  cursorily  at  this 
point  of  detail,  would  represent  the  broken,  curved,  and  unsymmetrical  nebula  in 
question  (lying,  as  it  does,  in  its  whole  extent,  clearly  out  of  the  line  of  junction  of 
the  two  stars  above  mentioned),  as  it  is  represented  in  the  earlier  of  the  two  figures; 
and  to  suppose  it  seen  as  in  1837,  and  yet  drawn  as  in  1*24,  would  argue  more  negli- 
gence than  I  can  believe  myself  fairly  chargeable  with. 

"There  is  another  point  on  which  considerable  stress  might  be  laid  were  I 
satisfied  that  the  earlier  diagrams  on  which  it  turns  were  done  with  sufficient  care. 
In  1837,  the  nebulous  spur  towards  the  end  of  the  great  proboscis,  which  terminates 
at  K  (No.  in)  [G.  P  B.  746],  certainly  was  neither  joined  to  the  proboscis  itself 
nor  directed  towards  the  star  A  (No.  135)  [G.  P  B.  843],  but  rather  towards 
a  point  about  one-third  of  the  distance  from  A  (No.  135),  to  C  (No.  126),  near 
to  where  there  is  a  small  star  16  m  (No.  131).  Now  I  find  two  diagrams,  one 
of  December  25,  1832,  the  other  of  November  25,  1834,  in  which  this  spur  is  repre- 
sented as  running  directly  from  A  to  E,  and  forming  a  complete  hook,  no  way  dis- 
joined from  the  proboscis.  But  the  chief  attention  on  the  first  of  these  -occasions  was 


56          MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

directed  to  the  magnitudes  and  situations  of  the  stars,  and  the  hook  seems  to  have 
been  only  roughly  sketched  in  as  a  novelty  to  be  further  noticed  in  future,  while  on 
the  last  it  was  only  very  faintly  indicated  in  a  diagram  of  the  stars  adjacent  to  0  Orionis 
on  all  sides,  preparatory  to  the  formation  of  chart  intended  to  take  in  both  z  Orionis, 
on  the  one  side,  and  C  Orionis  on  the  other,  which  was  subsequently  discontinued 


FIG.  22.     DE  Vico  AND  RONDONI  (1839-1841). 

(69)  "Still  less  can  we  insist,  as  evidences  01  change,  on  such  particulars  as  the 
curiously  notched  outline  of  the  "nebula  Mairanni"  about  the  star  ju  (No.  108) 
[G.  P.  B.  734],  now  for  the  first  time  represented;  or  on  the  intricately  rifted  and 
broken  state  of  the  frontal  and  occipital  region  of  the  principal  nebula.  I  ought  to 
mention  here  that  (owing,  no  doubt,  to  the  difficulty  of  properly  representing  on  paper 
and  by  lamp-light  an  object  of  the  kind)  I  find  a  good  deal  of  disagreement  in  respect 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  57 

of  the  number,  size,  and  distribution  of  the  portions  into  which  it  may  be  considered 
as  broken  up,  not  only  between  my  present  figure  and  Dr.  LAMONT'S,  but  between  my 
own  drawings  of  this  part  on  several  nights.  But  the  most  material  difference  between 
Dr.  LAMONT'S  figure  and  mine  consists  in  the  characteristic  forms  of  these  portions, 
which  he  represents  as  rounded  masses  more  or  less  detached  from  or  running  into 
each  other  and  into  a  general  nebulous  ground ;  while  in  all  my  later  drawings  the 
effect  is  rather  that  of  a  tolerably  uniform  surface  marked  with  branching  rifts  or 
channels  like  roads.  There  is  one  peculiarity  in  Dr.  LAMONT'S  figure  which  I  can  no 
way  reconcile  to  my  own  impressions,  viz,  the  strangely  different  form  and  magnitude 
which  he  assigns  to  the  "Sinus  Gentilii,"  from  what  I  have  always  found  it.  This  is 
a  point  which  I  trust  he  will  be  induced  to  re-examine." 

A  full  discussion  of  HERSCHEL'S  drawing  is  given  by  LIAPONOFF  and  STRUVE,  and 
will  be  best  understood  in  connection  with  their  observations. 

OBSERVATIONS  OF  DE  VICO  AND  RONDONI  (1839-1841). 

These  are  detailed  in  the  Memoirs  of  the  Roman  College  in  the  volumes  for  1839 
and  1 84 1 .  Fig.  20  gives  the  best  one  of  the  three  drawings.  It  was  made  by  M- 
ROVERE  and  probably  revised  by  P.  DE  Vico.  Attention  may 
be  called  to  the  comparative  faintness  of  the  region  E  (Index- 
Chart),  the  south  point  of  the  Huyghenian  region. 

i 

OBSERVATIONS  OF  KAISER  (1844). 

These  are  detailed  in  Die  Sterrenhemel,  vol.  ii,  Plate  iii, 
Fig.  i,  and  p.  538.  I  have  not  been  able  to  see  this  work, 
but  I  owe  to  Dr.  v.  DER  SANDE  BAKHUYSEN  a  copy  of  the  plate 

i  .   i    .        .  .      TT  FIG.  23.    KAISER,  1844. 

which  is  given  in  rig.  23. 

OBSERVATIONS  OF  COOPER  (1847). 

Through  the  kindness  of  Dr.  DOBERCK  I  have  received  an  exact  copy  made  by 
him  of  an  outline  sketch  of  the  nebula  made  by  COOPER  about  1847.  I*  ig  not  repro- 
duced here,  as  the  only  points  of  note  are : 

i  st.  E,  of  the  Index-Chart,  has  at  its  s.  p.  corner  a  curved  continuation  like  MES- 
SIER'S  E,  and  similar  in  outline  to  LASSELL'S  drawing  of  1862. 

2d.  Three  small  projections  are  shown  on  the  north  shore  of  T  (of  the  Index-Chart) 
similar  to  those  shown  in  HERSCHEL,  1824  (Fig.  19).  The  east  one  of  these  is  the 
longer,  the  west  one  the  shorter.  They  undoubtedly  refer  to  the  bridges  of  SCHROETER. 

3d.  At  the  spitz  (s.  f.  point  of  a  in  Index-Chart)  COOPER  has  a  small  star  marked 
No.  ii. 

4th.  An  oval  space  near  where  star  602  would  be  (it  is  not  laid  down)  marks 
some  special  region.  It  is  in  all  probability  intended  to  discriminate  the  blank  channel 

following  I. 

OBSERVATIONS  OF  LASSELL  (1847). 

In  February,  1847,  Mr.  LASSELL  made  some  studies  of  the  nebula  at  his  observ- 
atory, Starfield,  near  Liverpool.     An  oil-painting  of  this  date  was  presented  to  the 
App.  V 8 


58  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Royal  Astronomical  Society,  and  a  copy  also  is  preserved  at  Ray  Lodge,  his  resi- 
dence in  later  years.  These  paintings  were  poorly  copied  in  an  engraving  privately 
distributed,  a  portion  of  which  is  given  in  Fig.  24. 

The  full  title  of  the  oil-painting  which 
was  made  at  Burlington  House  from  LASSELL'S 
observations  is,  "The  Great  Nebula  of  Orion, 
with  its  Stars,  as  seen  in  February,  1847,  with 
the  Starfield  Equatorial  Reflector  of  24  inches 
aperture  and  242  inches  focus.  LASSELL." 

This  was  also  reproduced  in  NICHOL'S 
Architecture  of  the  Heavens,  p.  106,  Fig.  x. 

Little  need  be  said  of  this  in  this  place, 
except  to  call  attention  to  the  peculiarities  of 
light  and  dark  within  the  Huyghenian  region 
itself,  and  particularly  near  A  (Index-Chart). 

FIG.  24.     LASSELL,  1847.  f         .      -. 

As  a  drawing,  it  does  not  compare  with  the 
later  and  admirable  one  made  (1862)  in  Malta.     See  Fig.  31. 

OBSERVATIONS  OF  W.  C.  BOND  (1848). 

The  essential  parts  of  W.  C.  BOND'S  description  of  his  observations  with  the  Har- 
vard College  Refractor  in  1 848  are  extracted  below : 

"  All  such  parts  of  the  nebula  in  the  vicinity  of  the  trapezium  as  presented  definite 
outlines  susceptible  of  being  measured  were  referred  to  #'."  ....  "  The  stars 
Nos.  10,  12,  26,  and  27  [G.  P.  B.,  Nos.  567,  573,  647,  651]  mark  the  present  bound- 
aries of  the  Huyghenian  region  .  .  .  very  accurately No  10  [567]  is  situated 

close  on  the  preceding  edge  of  this  bright  region,  and  is  closely  followed  almost  in  the 
same  parallel  [meridian  f]  by  No.  12  [573],  a  star  of  the  i7th  magnitude,  the  latter 
being  within  the  boundary.  [I  must  believe  that  No.  1 1  m  G.  P.  B.  575,  marked  i8th 
magnitude,  is  here  meant,  in  spite  of  the  evidence  from  the  letter  of  the  text  to  the 
contrary.]  No.  27  [651]  is  as  nearly  as  is  possible  to  determine  with  our  telescope  on 
the  very  edge  of  the  following  side  ....  and  is  pretty  closely  preceded  by  No. 
26  [647]  of  the  1 7th  magnitude  within  the  bright  part*  ....  There  is  a  great 
diminution  of  the  light  in  the  interior  of  the  trapezium  but  no  suspicion  of  a  star.  Sir 
JOHN  HERSCHEL'S  drawing  shows  the  southern  termination  of  the  Huyghenian  region 
(E)  strongly  preceding  [0'],  whereas  I  have  repeatedly  laid  the  micrometer  wire  upon 
it,  and  have  found  it  to  be  of  the  same  right  ascension  as  [<9'].  The  difference  of 
declination  is  ....  161"."  "  The  bright  portion  of  the  Huyghenian  region  termi- 
nates abruptly  and  roughly  at  No.  50  [708]."  [That  is,  the  following  point  of  Q  is  in 
the  same  R.  A.  as  708,  or  Ja  =  1 50". 5.]  "The  preceding  side  of  the  Huyghenian  region 
in  [Sir  JOHN  HERSCHEL'S  figure]  has  the  light  gradually  softened  away  into  the  Regio 

*  In  Proc.  Amer.  Acad.,  i,  p.  326,  W.  C.  BOND  says  that  a  star  which  he  calls  h  91.  but  which  really  is  G.  P.  B.  651 
+  647  =  ad  75,  "has  hitherto  been  taken  for  a  single  star  of  the  I7th  magnitude.  This  is  double,  and  the  direction  is 
towards  6'  of  the  trapezium.  The  following  one  of  this  pair  [651]  is  as  precisely  as  possible  on  the  following  edge  of 
the  bright  part  of  the  nebula  at  the  bottom  of  the  Sinus  magnus"  The  last  phrase  identifies  the  stars  as  647  and  651. 
The  position  of  651  is  now  practically  the  same  as  in  1848. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  59 

Gentiliana.  I  here  see  a  strong  irregular  outline  extending  from  the  Sinus  Gentilii  to 
a  little  beyond  No.  10  [567]."  [This  is  quite  different  from  to-day,  when  J  is  a 
strongly  marked  mass,  nearly  all  of  which  is  dark  in  BOND'S  description,  and  it 
accounts  for  the  angle  of  position  of  the  occiput  in  his  drawing^—  147°. 5  against 
p=  136°  for  G.  P.  BOND'S  drawing  of  1865.]  A  further  discussion  of  these  observa- 
tions, occurs  in  connection  with  LIAPONOFF'S  and  STRUVE'S  measures. 

Fig.  25  is  a  copy  of  the  steel 
engraving  published  by  BOND. 

I  would  call  attention  to  the 
convex  outline  of  the  from  which 
BOND  first  correctly  laid  down. 
The  mass  A  just  preceding  the  tra- 
pezium is  also  faint.  The  other 
masses  are  well  terminated,  and 
appear  differently  disposed  from 
their  present  arrangement. 

Through  the  kindness  of  Pro- 
fessor PICKERING,  director  of  Har- 
vard College  Observatory,  I  have 
had  BOND'S  original  drawings  arid 
observations  in  my  possession  for 
some  months ;  and  although  I 
have  not  the  sketch  from  which 

,  i  •  •  j       T  i  FIG.  25.    BOND,  1848. 

this  engraving  was  made,  1  have 

another  nearly  completed  one.  On  this  A  is  considerably  brighter  than  in  Fig.  25.  E 
is  quite  bright,  and  there  is  a  suspicion  that  the  masses  within  the  Huyghenian  region 
were  not  laid  down  one  by  one  and  accurately,  but  dotted  in  to  represent  the  general 
effect  simply. 

From  other  original  sketches,  also  unpublished,  I  find : 

1847,  Dec.  13.  [Order  of   brightness  appears  to  be  D,  B,  A,  and  the  regions 
round  G,  H,  E  ;  region  round  F,  I.     These  estimates  are  rather  uncertain.] 

1848,  Jan.   17    [Order  of  brightness  appears  to  be  D ;  part  of  J?,  I;  G?,  HI, 
orF?;    Q,  N,  A;  north  parts  of  E.     These  estimates  are  again  rather  uncertain.] 
Large  completed  drawing  (no  date).     [Order  of  brightness  D,  E,  A.     These  masses 
are  plain,  and  others  are  indicated,  but  not  sufficiently  to  deduce  the  order  of  brightness 
intended  by  the  author.] 

OBSERVATIONS  OF  LIAPONOFF  AND  STRUVE  (1847-1851). 

§  20  [p.  68].  Observations  sur  differ entes  regions  de  la  nebuleuse. — La  nature  et  la  con- 
stitution physique  de  la  ne'buleuse  ne  pouvaient  devenir,  *\  cause  de  sa  position  australe 
et  sous  les  conditions  pen  favorables  de  ma  station,  un  objet  d'observations  detaillt'es 
et  rdgulieres.  L'extreme  difficult^  des  recherches  de  ce  genre  se  fait  dejt\  sentir  en 
comparant  superficiellement  les  resultats  obtenus  par  les  travaux  de  Sir  J.  HER- 
SCHEL  [1837],  M.  LAMONT  [1839],  et  M.  BOND  [1848].  En  examinant  ces  resultats, 


6o 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


on  est  le  plus  frapp^  par  la  grand  difference  qui  existe  dans  les  dessins  de  MM.  HER- 
SCHEL  et  BOND,  par  rapport  aux  formes  et  a  la  constitution  de  la  region  centrale,  la 

plus  lumineuse  et  la  plus  definie  de 
toutes  les  parties  de  la  nebuleuse.  II 
est  presque  impossible  de  concilier  sous 
ce  rapport  les  deux  dessins  sans  ad- 
mettre  la  supposition  d'un  changement 
considerable  qu'aurait  subi  cette  region 
dans  1'intervalle  e'coul<3  entre  les  ^poques 
des  deux  observations.  L'eclaircisse- 
ment  de  ces  discordances  appartient 
certainement  a  un  travail  futur  a  entre- 
prendre  dans  ce  but  special  a  1'aide  d'un 
telescope  de  force  suffisante  et  sous  des 
conditions  favorables  dans  un  degrd  pro- 
portionnd  Saris  pouvoir  affirmer  rien 
de  positif  sur  ce  sujet,  je  vais  cepen- 
dant  exposer  les  re'sultatsdes  etudes  com- 
paratives, que  -fai  eu  1'occasion  d'exe- 

FIG.  26.    O.  STRUVE,  1862.  , 

cuter  dans  le  courant  de  mes  observa- 
tions sur  les  dessins  des  trois  astronomes  nommes. 

§21  [p.  69].  Etudes  relatives  a  la  region  centrale  de  HuygTiens, — Cette  region 
m'a  constamment  paru  offrir  sur  ces  limites  des  formes  prononcees,  qui  se  laissent 
definir  par  des  lignes  a  pen  pres  droites.  La  planche  II  donne  une  representation 
graphique  de  sa  figure  g^nerale,  qui  est  d^finie  selon  mes  observations  par  une  ligne 
rompue  A,  B,  C,  F,  E,  G,  D.  Les  pointes  A,  B,  C,  etc.,  se  presentaient  dans  ma 
lunette  si  bien  termine'es,  que  j'ai  trouve  possible  de  fixer  leur  position  par  des  mesures 
microme'triques  avec  une  exactitude  assez  considerable. 

Les  resultats  d^finitifs  de  mes  mesures  sur  la  position  des  pointes  observers  s'eta- 
blissent  maintenant : 


A. 

D. 

AW 

Dw 

a  [628]  et  D 

-     9-1-3 

n 
-     39-9 

a  [628]  et  E 

—     87.6 

+     44-3 

•       . 

.    • 

a  [628]  et  G 

-     81.3 

+     18.4 

a  [628]  et  A 

-       4-4 

-   138.0 

+  o"  to  +  i" 

-   M5" 

a  [628]  et  F 

4-     26.5 

+     69.6 

.     . 

+     94"? 

a  [028]  et  C 

-t-     32.8 

+       3-1 

.     . 

.    .' 

a  [628]  et  K 

+    143-6 

+     7L5 

+    1  66" 

+     79" 

a  [628]  et  B 

+   146.5 

—      22.  0 

+   150" 

-     28" 

The  approximate   positions  of  these  points  from  Washington  observations  are 
given  for  comparison  in  the  columns  Aw  and  Dw. 

En  comparant  la  figure  que  mes  observations  assign^pt  a  la  region  de  HUYGHENS, 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  6 1 

avec  le  dessin  de  Sir  J.  HERSCHEL,  je  retrouve  ici  les  pointes  A,  C,  F  et  la  direc- 
tion AD,  la  position  de  la  pointe  A  dans  ce  dessin  parait  convenir  tres  exactement 
aux  resultats  de  mes  mesures.  D'un  autre  cot(j  les  pointes  B,  D,  G,  E  manquent 
dans  la  figure  que  donne  a  cette  region  Sir  J.  HERSCHEL.  Cependant  selon  .mes 
observations  la  pointe  B  [Q]  est  la  plus  prononce'e  par  une  condensation  particu- 
liere  de  la  matiere  n^buleuse  dans  son  voisinage,  nomme'ment  dans  un  espace  triangu- 
laire,  qui  attend  k  1'ouest  &  partir  de  cette  pointe.  Get  espace  lumineux  se  retrouve 
dans  le  dessin  de  HERSCHEL,  mais  au  lieu  de  se  terminer  par  une  pointe  il  se  prolonge 
&  Test  jusqu'k  la  region  de  MESSIER.  La  comparaison  avec  le  dessin  de  M.  BOND 
[1848]  donne  h  peu  pres  les  memes  r(3sultats,  on  n'y  trouve  que  les  pointes  A,  C,  F, 
la  pointe  B  [Q]  y  manque  comme  les  autres,  et  la  region  pres  de  B  y  est  plutot  faible, 
que  lumineuse.  La  forme  de  la  region  se  niontre  plus  conforme  k  mes  observations 
dans  le  dessin  de  M.  LAMONT.  Je  retrouve  ici  mes  pointes  A,  B,  C,  F  et  G.  Parmi 
les  mesures,  qu'avait  institutes  M.  LAMONT,  voy.  Obs.  Astr.  in  Spec.  Reg.  Monach.  InsL, 
vol.  XI  seu  VI,  p.  21  et  22,  les  suivantes  se  laissent  comparer  avec  les  miennes: 


P. 

d. 

d'. 

a  et  A 

180  27 

R. 

a 

A  et  D 

32i  53 

a  et  A  D 

7.027 

[  96.3] 

A  et  B 

50  521 

a  et  AB 

8.310 

[H3-9] 

B  et  C 

282     4 

Ay  ant  compare  les  distances  observees  par  M..  LAMONT  entre  plusieurs  e'toiles,  avec 
les  miennes,  j'ai  trouvd  la  valeur  angulaire  de  sa  vis  micrometrique  R  —  13". 745.*  Il 
en  resulte  que  les  distances  des  directions  AD,  AB  (occiput,  frons)  k  Fe'toile  &  Orionis, 
observers  par  M.  LAMONT,  sont:  q6".6  [96". 3]  et  114'' '.2  [ii3".9].  Par  un  calcul  je 
deduis  de  mes  mesures  les  m^mes  distances  =  96". 4  et  io6//.6.  Ainsi  les  differences 
de  nos  nisultats  sont: 

Liapounov—Lamont. 


rfP. 

ad. 

a  et  A 

+       82 

A  et  D 

-     255 

[+  o.i] 

A  et  B 

+       87 

[-  7.3] 

B  et  C 

+       26 

[Washington  observations  of  the  distance  of  occiput  from  &  (1877,  January  5) 
give  s—  io6x/.5(3)  LIAPONOFF-HOLDEN  =r  —  io/7.i. 

[p.  74].  Quant  a  la  constitution  physique  de  la  region  de  HUYGHENS,  mes  obser- 
vations indiquent  d'une  maniere  certaine  1'existence  r^elle  de  plusieurs  centres  de  con- 

*This  value  was  really  13".  7036  according  to  a  letter  from  Dr.  LAMONT  to  Professor  HUBBARD,  U.  S.  N.,  and  hence 
LAMONT'S  measures,  as  here  given,  are  too  large  in  the  ratio  of  1,000  to  997.  I  have  corrected  the  more  important  of 
1  hese. 


62 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


densation  dans  les  limites  de  cette  region.  La  planche  II  represents  en  or0,  a0,  &0,  c0, 
d0,  A0  six  centres  d'une  forte  accumulation  de  la  lumiere  diffuse,  dont  je  puis  affirmer 
1'existence  reelle  et  dont  je  suis  parvenu  a  fixer  la  position  par  des  mesures  microme- 
triques.  Le  centre  a0  [in  F]  fut  remarque  des  le  commencement  de  mes  observations 
en  1 848  et  je  le  regardais  a  cette  (3poque  comme  une  petite  etoile  entouree  d'une  atmos- 
phere nebuleuse.  En  1849  j'avais  remarqu^  une  condensation  analogue  autour  des 
centres  a0  et  &0  [in  D  and  G].  Cependant,  des  connaissances  plus  positives  sur  la  dis- 
tribution de  la  matiere  nebuleuse  dans  la  region  de  HUYGHENS  ne  furent  acquises  qu'a 
1'epoque  de  la  derhiere  seVie  de  mes  observations,  en  1851.  Avant  que  je  fus  par- 
venu a  reconnaitre  la  vraie  nature  de  cette  distribution,  toute  la  region  me  prt3sentait 
ordinairement  des  apparences  qui  variaient  d'un  jour  a  1'autre  selon  les  circonstances 
atmospheriques.  Meme  a  1'epoque  de  la  demiere  serie,  la  plus  favorisee  par  I'e'tat 
atmosphe'rique,  j'e'tais  encore  longtemps  ind^cis  par  rapport  a  ce  sujet  Ainsi  je  trouve 
dans  mon  journal  la  note  suivante,  inscrite  le  24  fevrier  1851  :  "  J'ai  cherch(^  en  vain 
dans  la  region  de  HUYGHENS  la  repartition  r^guliere  de  la  ndbulosit^  en  masses  globu- 
laires,  indique'e  par  MM.  HERSCHEL  et  LAMONT,  bien  que  je  crois  avoir  remarqu^ 
quelque  chose  d'analogue  anterieurement.  Toute  la  region  me  parait  offrir  aujourd'hui 
les  apparences  d'une  surface  liquide  qui  se  trouve  en  mouvement  ondulatoire  rapide." 
Quelques  jours  apres  j'ai  reconnu  d'une  maniere  certaine  les  masses  de  HERSCHEL  et 
ce  r^sultat  est  inscrit  dans  mon  journal  dans  les  termes  suivants:  "  Le  2  mars  1851. 
Aujourd'hui  je  vois  distinctement  dans  la  region  de  HUYGHENS  les  masses  globulaires 
de  HERSCHEL,  trois  dans  la  partie  australe,  A0,  «0,  d0  [El,  F,  I],  et  deux  un  peu  plus  au 
nord,  &0,  c0  [G,  H].  En  a0  [F]  je  supposais  auparavant  une  petite  e"toile  n^buleuse, 
probablement  c'est  une  masse  analogue,  qui  presente  une  condensation  tres  forte  pres 
du  centre."  Depuis  ce  jour  ont  commence  les  observations  positives  sur  la  re'gion  de 
HUYGHENS  et  jusqu'a  la  fin  de  la  seYie,  en  mois  d'avril,  je  voyais  constamment  et  sans 
difficult^  la  distribution  mentionri(3e  de  la  matiere  n^buleuse  en  masses  globulaires 
condense'es. 

Les  mesures  microme'triques,  ex^cute'es  par  moi  pour  fixer  la  position  des  centres 
reconnus  de  condensation  par  rapport  a  &  Orionis,  sont:  [Omitted]. 

Ayaiit  corrigt^  ces  mesures  pour  1'effet  de  la  refraction,  j'en  tire  les  coordonndes 
suivantes : 


Centre. 

A 

Aw 

D 

Dw 

</o      .      .      .      . 

-     34-5 

[-  33.0] 

—     81.9 

[-  67.5] 

These  are  G.  P.  BOND'S  co-ord.  of  602  for  1857.0. 

^o      .... 

+      18.5 

+   16 

-     63.3 

-  65 

a0      .      .      .      . 

+     26.1 

[+   29..  4] 

-H     55-5 

[+  47-8] 

G.  P.  B.'s  651. 

a0      .      .      .      . 

+     28.6 

+   27 

-     97-4 

—    10.2 

Co         .... 

+      51-8 

•      • 

-     66.3 

•       • 

[Aw  and  L)w  give  approximate  positions  from  Washington  observations.] 


Quant  a  la  masse  A0  (E),  sa  position  est  donn^e  par  celle  de  la  pointe  A  (apex)  qui 
en  forme  le  sommet  austral. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  63 

Au  sujet  de  la  nature  des  masses  nebuleuses  condenses  autour  de  centres,  je 
trouve  dans  les  notes  de  mes  journaux  d'observation,  les  informations  suivantes: 

L'intensite  de  lumiere  dans  les  masses  «0,  «0,  &0,  et  c0  [D,  F,  G,  H]  est  si  grande 
qu'elles  furent  encore  visibles  par  une  illumination  tres-forte  des  fils  micromdtriques 
sans  1'usage  des  mod^rateurs.  Ces  masses  m'avaient  pre*sente  a  plusieurs  occasions  des 
ressemblances  frappantes  avec  des  amas  d'etoiles.  Le  caractere  stellaire  s'est  prononce 
d'abord  dans  la  masse  la  plus  lurnineuse  a0  [D],  dont  1'apparence  me  conduisait  depuis 
constamment  a  1'idee  d'une  agglomeration  de  petites  Voiles  condenses.  L'intensite 
de  la  masse  a0  [F]  me  parut  quelquefois  presque  egale  a  celle  de  la  masse  a0  [D],  et  son 
apparence  porte  les  caracteres  de  constitution  stellaire  k  peu  pres  avec  la  meme 
Evidence.  Dans  la  masse  b0  [G]  j'ai  cru  pouvoir  remarquer,  a  cote  des  indices  d'un 
etat  stellaire,  la  presence  d'une  matiere  n^buleuse  insoluble ;  du  moins  il  est  sur  que 
la  condensation  de  lumiere  y  est  plus  faible  que  dans  les  autres  masses.  Le  caractere 
des  amas  stellaires  est  plus  difficile  a  reconnaitre  dans  la  masse  c0  [H],  dont  1'intensite 
me  parut  encore  plus  faible  que  celle  de  60  [G].  Quant  h  la  masse  A0  [E]  situe^e  au 
sommet  austral  de  la  region,  elle  est  beaucoup  plus  grande  que  les  autres,  mais  elle  ne 
montre  ni  le  caractere  des  amas  stellaires,  ni  une  condensation  assez  forte,  et  sa  lumiere 
est  d'une  intensity  consideVablement  plus  faible.  Enfin  dans  la  nebulosite  du  centre 
d0  [I]  plus  intense  que  celle  de  A0  [E],  je  reconnaissais  les  indices  de  constitution 
stellaire  a  peu  pres  dans  le  meme  degrd  de  deVeloppement  que  dans  la  masse  c0  [H]. 

En  rapprochant  ces  r^sultats  de  mes  Etudes  a  ceux  des  travaux  antdrieurs,  je 
trouve  d'abord  que  mes  centres  de  condensations  A0,  «0,  b0,  c0  [E,  D  .  .  .  .]  sont  iden- 
tiques  avec  les  agglomerations  globulaires,  indiquees  dans  le  dessin  de  Sir  J.  HERSCHEL. 
Cependant  il  existe  une  difference  importante  entre  les  r^sultats  de  nos  observations 
par  rapport  h  ces  masses.  Dans  le  dessin  de  Sir  J.  HERSCHEL  les  diffeVentes  masses 
sont  toutes  de  la  meme  intensity  et  en  outre  les  masses  isoiees  ne  pre*sentent  que  des 
indices  tres-faibles  de  condensation  vers  le  centre  et  ressemblent  plutot  aux  disques 
arrondis  d'un  ^clat  uniforme  des  ndbuleuses  plandtaires.  La  forme  de  la  masse  d0  [I] 
est  beaucoup  plus  irrdguliere  que  d'apres  mes  observations  et  on  y  voit  le  meme  carac- 
tere d'un  dclat  uniforme  dans  toute  son  etendue.  Quant  &  la  position  des  masses  a0, 
fc0,  c0  [F,  G,  H],  elle  ne  s'accorde  pas  assez  avec  les  resultats  de  mes  mesures  micro- 
metriques.  Enfin  la  masse  a0  [D]  n'existe  pas  du  tout  dans  le  dessin  de  HERSCHEL,  on 
n'y  remarque  qu'une  faible  condensation  de  lumiere  a  peu  pres  k  1'endroit,  ou  j'ai  vu 
cette  masse  brillante.  D'un  autre  cote  les  journaux  de  mes  observations  ne  font 
aucune  mention  de  la  masse  arrondie,  qui  est  representee  dans  le  dessin  de  Sir  J. 
HERSCHEL  en  connexion  immediate  avec  la  nebulosite  situee  entre  ma  pointe  C  [in  T 
near  654]  et  la  masse  c0  [H]. 

II  est  plus  difficile  de  concilier  mes  observations  sur  la  constitution  de  la  region 
de  HUYGHENS,  dans  sa  partie  australe,  avec  le  dessin  de  M.  BOND.  Le  nombre,  la 
position  et  les  formes  des  masses,  qui  se  trouvent  dans  son  dessin,  ne  conviennent  pas 
k  la  repartition  reguliere  de  la  nebulosite,  que  j'avais  observee.  Cependant  dans  la 
partie  boreale  de  la  region  je  trouve  chez  M.  BOND  la  masse  a0  [in  D],  qui  manque  au 
dessin  de  HERSCHEL.  II  est  vrai  que  M.  BOND  lui  donne  1'apparence  d'un  disque  d'un 
eclat  uniforme,  mais  il  declare  dans  son  memoire  qu'il  a  remarque  dans  toutes  les 


64 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


masses  de  la  region  Huyghenienm  le  caractere  stellaire  et  qu'il  croyait  meme  pouvoir 
distinguer  dans  des  circonstances  favorables  les  dtoiles  qui  les  constituent. 

Compare'es  aux  observations  de  M.  LAMONT,  les  notres  offrent  une  ressemblance 
frappante.  On  retrouve  dans  son  dessin  les  masses  A0,  a0,  &0,  c0,  et  d0,  leur  position 
s'accorde  de  tres  pres  avec  celle  que  leur  assignent  mes  mesures  et  le  caract£re  de 
condensation  y  est  exprime'  d'une  maniere  analogue.  II  n'existe  qu'une  seule  dis- 
cordance entre  nos  re'sultats,  nomme'ment  dans  I'intensitt^  relative  des  masses  observe'es. 
Selon  M.  LAMONTI  les  masses  A0  et  d0  [E  and  I]  sont  consideYablement  plus  lumi- 
neuses  que  les  autres,  ce  qui  semble  indiquer  un  ddveloppement  plus  rapide  dans  les 
masses  a0,  b0,  c0  [F,  Gr,  H],  qui  sont  a  1'dpoque  actuelle  d'une  intensite'  superieure 
d'apres  mes  observations.  En  consideYant  que  d'ailleurs  nos  resultats  s'accordent  tres 
bien  par  rapport  a  la  distribution  de  la  matiere  nebuleuse  dans  la  partie  australe  de  la 
region,  il  parait  tres  remarquable  que  la  masse  a0  [D]  est  represented  dans  le  dessin 
de  M.  LAMONT  comme  une  masse  oblongue,  qui  ne  montre  pas  de  condensation  vers  un 
centre.  Ce  centre  se  serait  done  developpe'  consideYablement  entre  les  e'poques  de  nos 
observations,  ce  qui  parait  confirm^  par  les  dessins  de  Sir  J.  HERSCHEL  et  de  M.  BOND, 
dont  les  e'poques  s'accordent  respectivement  avec  celles  des  observations  de  M.  LAMONT 
et  les  miennes. 

Les  mesures,  exe'cute'es  par  M.  LAMONT  sur  la  position  des  masses  condensees,  sont : 


P. 

d. 

d. 

a  et  a0 

[628  et  F] 

165  42.0 

105.70 

[105.4] 

a  et  ^o 

[628  et  G] 

163  54-3 

. 

a  et  fo 

[628  et  H] 

140  46.5 

86.46 

[86.2] 

Kn  supposant  que  ces  mesures  se  rapportent  egalement  aux  centres  des  masses 
observe'es,  nous  trouvons  les  differences  suivantes  avec  nos  r^sultats : 

Liaponoff-Lamont. 


dP. 

dd. 

a  et  «o 

—   118.0 

—     4.20 

[-    3-9] 

a  et  bo 

—      10.3 

c  et  fo 

+     79-5 

—     2.42 

[-     2.2] 

[p.  77.]  Outre  les  centres  mentionne's  de  condensation  eVidente  j'ai  remarque' 
encore,  qu'aux  pointes  boreales  G,  E  [B,  C],  il  y  a  lieu  de  supposer  un  ddveloppement 
recent,  encore  peu  prononce,  de  deux  centres  pareils.  La  matiere  n^buletise  autour  de 
ces  pointes  m'avait  pr^sente  quelquefois  des  indices  d'un  etat  stellaire ;  cependant  j'ai 
trouve'  une  difficulte'  extreme  de  parvenir  a  un  jugement  definitif  sur  sa  nature  et  mes 
observations  ne  donnent  a  ce  sujet  aucun  r^sultat  positif.  J'ai  dej{\  mentionnci  plus 
haut  que  ces  pointes  manquent  dans  les  dessins  de  MM.  HERSCHEL  et  BOND  ;  dans 


MONOGRAPH  OF  THE-  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  65 

celui  de  M.  LAMONT  je  trouve  autour  de  la  pointe  G  [north  end  of  B]  une  masse  con- 
densed d'une  e"tendue  considerable,  mais  la  pointe  E  [in  C]  y  manque  aussi. 

A  cote  des  masses  arrondies  qui  pr^sentent  des  centres  marques,  j'ai  observe*  dans 
la  region  de  HUYGHENS  encore  deux  masses,  qui  montrent  un  genre  particulier  de  con- 
densation suivant  certaines  directions.  La  premiere  est  situee  a  1'ouest  de  la  pointe 
B  [in  Q]  dans  un  grand  espace  triangulaire  entre  B  [Q],  C  [inT]  et  c0  [H].  A  partir 
de  B  [Q]  jusqu'a  1'etoile  en  [671]  on  remarque  une  condensation  tres  considerable  entre 
les  directions  BA  et  KCfrons,  QPR.  Plus  loin  a  1'ouest  vers  C  [inT]  et  c0  [H]  cette 
condensation  s'affaiblit  successivement.  L'^clat  de  la  masse  dans  le  voisinage  imme'diat 
de  B  [Q]  me  parut  quelquefois  egal  a  celui  du  centre  brillant  a0  [D].  La  seconde 
masse  se  trouve  a  1'ouest  de  1'espace  noir,  qui  entoure  les  Voiles  du  trapeze  [V].  Elle 
prdsente  la  forme  d'un  demi-anneau  circulaire,  qui  se  termine  du  cote"  austral  k  peu 
pres  au  parallele  de  la  masse  60  [G]  et  du  cote  boreal  entre  les  pointes  D  et  G.  Dans 
toute  I'etendue  de  cet  anneau  on  voit  une  condensation  prononce'e  au  milieu  de  son 
^paisseur,  et  sa  region  moyenne  entre  1'dtoile  '&  Orionis  et  le  point  R  [near  608] 
possede  un  eclat  tres  intense.  Pendant  mes  Etudes  sur  ces  masses  brillantes  j'ai  cru 
aussi  remarquer  dans  leurs  parties  condensees  des  indices  de  constitution  stellaire. 
[The  Hemicyclium  Liaponovii  is  here  described-;  it  is  made  up  of  parts  of  I,  of  L,  and 
of  A.] 

Dans  le  dessin  de  Sir  J.  HERSCHEL  je  ne  retrouve  que  la  premiere  de  ces  deux 
masses  lumineuses.  Elle  y  montre  une  condensation  assez  forte  et  pr&ente  la  meme 
forme  triangulaire,  cependant  au  lieu  de  se  terminer  par  une  pointe,  comme  je  1'avais 
observe'e,  elle  se  prolonge  a  Test  dans  la  re'gion  de  MESSIER.  Quant  a  la  masse  annu- 
laire,  situe*e  a  1'ouest  du  trapeze,  elle  ne  se  retrouve  que  dans  le  dessin  de  M.  LAMONT, 
et  1'accord  de  nos  rdsultats  par  rapport  a  la  forme,  la  position  et  rintensite*  de  cette 
masse  est  a  peu  pres  parfait.  D'apres  les  observations  de  M.  LAMONT  la  masse  trian- 
gulaire est  plus  faible  et  moins  etendue  dans  le  sens  du  cercle  de  de'clinaison ; 
cependant  elle  possede  dans  son  dessin  un  ^clat  ^gal  avec  la  masse  «0  [D]. 

Les  intervalles  entre  les  masses  globulaires  A0,  «0,  &0,  c0,  d0  [E,  F,  G,  H,  I,  etc.], 
de  meme  que  les  regions  situdes  immediatement  au  sud  et  au  nord  de  1'espace  noir, 
qui  entoure  les  t'toiles  du  trapeze  [V]  sont  d'apres  mes  observations  d'une  intensite* 
beaucoup  plus  faible  en  comparaison  avec  les  regions  luniineuses  environnantes.  Dans 
ce  pointe  je  suis  d'accord  avec  les  observations  de  Sir  J.  HERSCHEL  et  de  M.  LAMONT, 
a  cette  exception  pres,  que  la  condensation  au  nord  de  1'espace  noir  [V]  est  encore 
assez  considerable  dans  les  dessins  de  ces  astronomes,  tandis  qu'elle  m'a  paru  extreme- 
ment  faible. 

Pendant  mes  Etudes  sur  la  region  de  HUYGHENS  j'ai  reconnu  1'existence  certaine 

d'un  espace  tres  sombre  dans  sa  partie  occidentale  [Sinus  Lamontii].     Cet  espace  est 

entoure  par  la  masse  brillante  annulaire,  dont  j'ai  parle*  plus  haut,  et  se  trouve  en 

communication  avec  la  baie  noire  de  LE  GENTIL,  qui  semble  peneVer  en  dedans  de 

APP.  Y 9 


66 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


la  region  de  HUYGHENS,  en  interrompant  la  limite  AD  (occiput)  entre  les  pointes  A  et 
D.  Les  positions  des  pointes  A  et  R  comme  elles  resultent  de  mes  mesures  corrig^es, 
sont: 


Point. 

A. 

D.       P. 

,/. 

Err.  prob. 

A 

-  61.5 

i 

—   76.4  :    2l8  41 

97-9 

/   a 
18  0.6 

•  R 

-  29.5 

—   28.6     225  46 

4I.O 

60  0.7 

[R  is  then  not  far  from  G.  P.  B.  601,  A  =  —  36"  ;  D  =  —  31",  1857.0.] 

Le  fond  de  1'espace  A,  RD  (Sinus  Lamontii)  m'a  paru  quelquefois  tout-a-fait  noirJ 
cependant  en  rdsumant  les  notes  qui  se  trouve  dans  mes  journaux  au  sujet  de  sa 
nature,  je  tire  la  conclusion,  qu'il  doit  6tre  plus  lumineux  que  la  baie  de  LE  GENTIL. 

En  revenant  aux  travaux  anterieurs,  je  ne  trouve  cet  espace  sombre  que  dans  le 
dessin  de  M.  LAMONT.  La  region  qui  correspond  k  sa  position  dans  le  dessin  de  Sir 
J.  HERSCHEL  offre  au  contraire  une  condensation  considerable  de  matiere  nebuleuse. 
De  m6me  la  ligne  AD  se  distingue  d'apres  M.  BOND  par  une  forte  condensation  dans 
toute  son  etendue.  II  parait  tres  difficile  de  donner  une  explication  satisfaisante,  com- 
ment cet  espace  si  sombre,  aperc.u  par  M.  LAMONT,  ne  fut  pas  remarquc,  dans  les  con- 
ditions atmospheriques  plus  avantageuses  oil  se  trouvait  Sir  J.  HERSCHEL  h,  la  meme 
epoque.  [The  continuation  of  the  [line  of  the  occiput  across  the  space  A  D  received 
the  particular  attention  of  Gr.  P.  BOND,  and  he  also  declares  that  it  certainly  exists.  It 
is  so  in  Washington  observations.] 

Enfin,  1'espace  qui  environne  les'etoiles  du  trapeze  m'a  paru  parfaitement  noir  et 
depourvu  de  tout  indice  de  matiere  n^buleuse.  II  est  tres  possible  cependant  que 
1'edat  des  etoiles  brillantes  du  trapeze  efface  les  faibles  lueurs  d'une  nebulosite"  rare'fie'e, 
qui  se  trouve  dans  leur  voisinage  imme'diat.  Sous  ce  point  de  vue  je, serais  a  pen 
pres  d'accord  avec  les  re'sultats  de  Sir  J.  HERSCHEL  et  de  M.  BOND.  Mais  il  existe 
entre  nos  observations  une  difference  essentielle,  en  ce  que  d'apres  mes  observations 
1'espace  sombre  s'dtend  conside'rablement  en  dehors  des  etoiles  du  trapeze.  La  dis- 
cordance sous  ce  rapport  est  encore  plus  grande,  si  on  compare  mes  observations  avec 
celles  de  M.  LAMONT.  Pour  fixer  la  position  et  1'etendue  de  1'espace  U,  T,  V,  X,  Y,  W 
[bounding  line  of  V],  que  je  voyais  sombre,  autour  des  etoiles  du  trapeze,  j'ai  execute 
la  mesure  des  angles  et  des  distances  entre  &  Orionis  et  les  pointes  qui  limitent  cet 
espace.  Apres  les  reductions  necessaires  j'en  tire  les  differences  suivantes  en  ascension 
droite  et  en  dedinaison  par  rapport^a  &  Orionis : 


Point. 

A. 

D. 

T 

-     35-2 

-1-      22.  0 

U 

-       1.7 

—       10.  1 

y 

+       3-1 

+      34-7 

X 

+     15-2 

+       13-0 

W 

+     26.2 

+         O.I 

67 

[In  the  Washington  observations  the  various  points  and  angles  do  not  appear, 
but  rather  rounded  contours,  and  any  measures  made  upon  them  would  be  illusory. 
The  4d  of  the  north  boundary  can,  however,  be  accurately  fixed.  It  is  -f-  24".o, 
which  agrees  tolerably  with  the  measures  of  T  above.] 

[p.  79.]  On  voit  que  1'espace  en  dehors  de  la  ligne  qui  joint  les  Voiles  les  plus 
brillantes  a,  c  [628,  640]  fut  observe"  moins  e*tendu,  que  du  c6te*  des  e'toiles  plus  faibles 
ft,  d  [619,  624].  II  est  done  probable,  que  le  ph^nomene  n'est  pas  produit  par  le  seul 
effet  du  contraste.  [This  observation  agrees  with  Washington  observations,  and  with 
.conclusions  drawn  from  them,  and  it  is  an  important  point  in  regard  to  the  question 
of  the  connection  of  the  nebula  with  the  stars.  I  believe  it  has  been  remarked  by  no 
one  except  LIAPOXOFF.] 

Le  resultat  le  plus  certain  de  mes  observations  sur  la  region  de  PICARD  est  celui 
de  1'existence  dans  la  grande  baie  noire  B,  C,  F,  K  [Sinus  magnus]  d'une  masse 
.pyramidale  [pom  Scliroeteri\  tres  faible,  situee  dans  la  direction  des  e'toiles  k  et  e  [669 
and  685],  et  qui  se  trouve  en  connexion  avec  la  limite  boreale  F,  K  [south  edge  of  a\. 
J'ai  vu  cette  masse  en  tout  conforme  au  dessin  de  Sir  J.  HERSCHEL,  k  1'exception  pres 
que  j'avais  observe'  encore  £i  son  extre'mite'  australe  une  petite  masse  condensee,  qui  prd- 
sente  des  indices  d'un  e*tat  stellaire.  La  position  de  ce  petit  et  faible  amas  par  rapport 
h  &  Orionis  suit  de  mes  mesures  corrige'es  : 

A.  ,   D. 

ffo         +  7i"-9         +  42"-3 
[ffo         H-  7  7" -6         +  40". 8         Washington  observations.] 

A  1'exception  de  la  masse  'pyramidale,  1'espace  B,  C,  F,  K  [Sinus  magnus]  me 
parut  en  general  sombre  et  de*pourvu  de  matiere  ne'buleuse.  Une  seule  fois,  le  4  mars 
1851,  j'ai  cm  reconnaitre  le  long  de  sa  limite  australe  C,  B,  Q,  P,  R,  la  serie  de  dents 
observers  par  HERSCHEL.  Cependant  il  m'a  e'te'  impossible  de  compter  le  nombre  et  de 
fixer  la  position  de  ces  dents,  que  j'ai  indique'es  dans  mon  dessin  d'apres  les  observations 
de  HERSCHEL. 

[p.  80.]  J'ai  deja  mentionnc;  plus  haut,  que  la  limite  AB  [from]  est  prolonged  dans 
le  dessin  de  Sir  J.  HERSCHEL  au  dela  de  la  pointe  B  [Q]  dans  la  region  de  MESSIER- 
Selon  mes  observations,  la  ndbulosite  situe'e  entre  les  pointes  B  et  L  est  d'une  nature 
tres  diffeVente  de  celle  qui  est  propre'aux  masses  ndbuleuses  condensees  dans  la  region 
de  HUYGHENS.  C'est  urie  masse  confuse  d'une  formation  peu  avancde,  qui  ne  prdsente 
pas  de  formes  de'finies  et  se  confond  avec  la  region  subndbuleuse.  Sous  ce  rapport  je 
suis  parfaitement  d'accord  avec  les  observations  de  M.  BOND.  Les  deux  branches 
lumineuses  de  la  region  de  MESSIER  commencent  selon  moi  au  pointe  L,  dont  la  posi- 
tion par  rapport  a  &  Orionis j&st  d'apres  mes  mesures  microme'triques : 


Quant  a  la  nature  de  la  lumiere  dans  les  deux  branches  de  MESSIER,  je  1'ai 
trouve  analogue  k  celle  des  nuages  nornme's  cyrrus. 

Dans  la  region  subn^buleuse  j'avais  reconnu  les  caracteres  de  radiation  observe'e 
par  M.  BOND;  cependant  j'ai  trouvc^  une  difficult^  extreme  de  fixer  le  nombre  et  la 


68  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

direction  des  raies  sombres  dont  elle  est  traversee,  a  cause  de  la  faiblesse  que  la 
lumiere  de  cette  region  presentait  dans  ma  lunette.  J'ai  indique  ce  caractere  dans 
mon  dessin  d'apres  les  observations  de  M.  BOND. 

La  region  de  LE  GENTIL  consiste  d'un  espace  sombre,  qui  communique  en  A,  D 
avec  un  espace  pareil  de  la  re'gion  de  HUYGHENS,  et  d'une  masse  reguliere  d'un  eclat 
assez  prononce',  qui  se  rdunit  en  D,  D  E  (K  and  /?)  avec  la  re'gion  de  HUYGHENS.  La 
limite  inteYieure  de  cette  masse  reguliere  est  un  arc  D,  Sh0  [border  of  ft  and  x].  La 
lumiere  s'affaiblit  peu  a  peu  a  partir  de  cet  arc  vers  1'occident.  Du  c6t^  bore'al  en  e0 
j'ai  observe*  une  petite  masse  condensed  de  nature  stellaire.  Un  amas  analogue, 
mais  plus  petit,  fut  reconnu  a  1'extremite'  australe  en  L0.  La  lumiere  qui  extend  de  cet 
amas  a  1'ouest,  pr(isente  la  forme  d'une  queue,  &  peu  pres  comme  dans  le  dessin  de  Sir  J. 
HERSCHEL.  La  limite  occidentale  de  toute  la  re'gion  se  trouve  un  peu  h  1'ouest  de  I'e'toite 
v  [558],  ou  sa  lumiere  affaiblie  se  confond  avec  les  lueurs  de  la  ne'bulosite'  environnante 
Malgre*  tous  mes  efforts  je  n'ai  pas  retrouve"  1'espace  sombre  qui  fut  observe  par  M. 
BOND  entre  le  point  S  [not  marked  in  figure]  et  1'e'toile  v  [558]  ;  au  contraire  j'ai  vu 
un  affaiblissement  graduel  de  la  masse  ne*buleuse  h  partir  de  la  limite  orientale  vers 
1'occident,  sans  aucun  changemerit  brusque  d'intensite*,  qui  aurait  pu  expliquer  1'espace 
noir  de  M.  BOND. 

II  me  reste  k  dire  que  la  distribution  de  la  matiere  n^buleuse  dans  les  regions  de 
FOUCHY  et  de  GODIN  s'accorde  en  ge'ne'ral  avec  les  observations  de  Sir  J.  HERSCHEL. 

EXTRACTS   FROM   THE   MEMOIR   OF   DIRECTOR   OTTO   VON    STRUVE. 

[p.  97.]  II  parait  que  les  observations  de  SCHROETER,  faites  k  Lilienthal  dans  les 
anne*es  1774  a  1779,  n'ont  pas  attire'es  toute  1'attention  qu'elles  me'ritent.  Elles  sont 
publie'es  dans  un  appendice  aux  "Aphroditographische  Fragmente"  et  dans  la  ire  sec- 
tion des  "Neueste  Beitrage  zur  Erweiterung  der  Sternkunde,"  Gottingen,  1800.  On 
y  lira  avec  inte'ret  que  des  changements  dans  la  distribution  et  I'e'clat  de  la  matiere 
ne*buleuse,  dont  on  verra  plus  bas  qu'ils  sont  note's  par  moi,  ont  et^  apercus  tout  h  fait 
de  la  meme  maniere  par  SCHROETER  et  son  aide  HARDING.  Cet  accord  est  d'autant  plus 
surprenant  que  je  n'ai  pris  connaissance  des  observations  de  Lilienthal,  que  quatre  ans 
apres  avoir  remarque*  les  dits  changements.  De  differents  cote's  on  a  reproche'  h  SCHROE- 
TER que  dans  ses  e'crits  il  s'est  laissti  quelquefois  entrainer  par  1'imagination ;  mais  ces 
reproches  concernent  surtout  les  conclusions  qu'il  tire  de  ses  observations,  pas  les  obser- 
vations elles-memes,  qui  sans  doute  ont  e'te'  faites  toujours  de  bonne  foi.  .  Personne 
n'oserait  attaquer  sa  since'rite',  si  meme  on  voulait  admettre  que  ses  observations  ont  e'te' 
en  partie  le  produit  de  deceptions  optiques  ou  d'illusions.  Heureusement  dans  notre 
cas  les  observations  sont  de  nature  que  la  supposition  d'une  deception  optique  est  tout 
a  fait  hors  de  question.  ***** 

1856,  Nov.  8.  Le  Sinus  Gentilii  avec  la  baie  qui  s'etend  de  lui  dans  la  re'gion 
HuygJienienne  vers  le  trapeze  [Sinus  Lamontii~]  est  le  mieux  repr^sente  dans  le  dessin 
de  M.  LIAPONOFF.     Cette  baie  ne  cede  pas  en  noirceur  au  Sinus  Gentilii. 

Dans  les  dessins  de  HERSCHEL  cette  baie  n'est  pas  indiqude,  et  non  plus  dans  celui 
de  BOND,  mais  elle  se  trouve  aussi  dans  le  dessin  de  M.  LAMONT.  *  *  *  * 

1857,  March  18.  La  baie  LAMONT,  aux  bords  de  laquelle  se  trouvent  les  deux 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  69 

Voiles  51  et  57  [567,  575],  est  aujourd'lmi  remplie  de  matiere  ne*buleuse,  de  sorte 
qu'elle  ne  contraste  que  tres  pen  avec  les  autres  parties  de  la  region  Huyghenienne, 
tandis  que  le  Sinus  Gentilii  lui-meme  est  parfaitement  sombre.  Le  pont  SCHROETER 
pr&ente  aujourd'lmi  un  ^clat  uniforme,  sans  lumiere  concentre  vers  le  milieu.  Les 
dtoiles  76,  80  et  84  [652,  657,  663]  se  trouvent  dans  un  lac  noir  de  forme  oblongue 
[lacus  Lasselii],  89  [68 1]  est  dejk  entouree  de  matiere  ne*buleuse  assez  dense  [in  a]. 

1857,  March  20.  An  sud  du  trapeze  deux  masses  globulaires  se  pre"sentent 
aujourd'hui  tres  distinctement  dans  la,  region  Huyghenienne.  Des  mesures  microme'- 
triques  donnent  les  positions  suivantes  de  ces  masses  par  rapport  a  c  [628]  : 

e=   68.0  Pz=  171.7     (60  =  G) 

106.5  164.2     (a»:=F) 

Evidemment  la  premiere  de  ces  masses  est  identique  avec  celle  que  M.  LIAPONOFF 
a  designee  par  &0  [G],  la  seconde  avec  a0  [F].  Cependant  les  differences  de  8°  dans 
I'angle  de  position  du  premier  objet  et  de  5". 4.  dans  la  distance  de  1'autre,  sont  assez 
surprenantes.  Mais  ce  qui  me  frappe  encore  plus,  c'est  que  dans  mon  journal  j'ai 
d^signe*  le  premier  objet  comme  la  masse  la  plus  precise  et  la  plus  luisante  de  toute  la 
region,  et  que  son  intensite  surpasse  consideYablernent  celle  de  1'autre  objet  qui,  de 
son  cot^,  est  plus  dtendu  que  le  premier — tandis  que  M.  LIAPONOFF  dit  exactement  le 
contraire  par  rapport  2i  I'lntensite*  des  deux  masses  &0  et  a0  [G,  F]. 

1857,  March  24.  Dans  les  environs  de  75  [647]  il  y  a  aujourd'hui  une  conden- 
sation tres  forte  de  matiere  ndbuleuse.  Aussi  le  pont  SCHROETER  pre*sente  aujourd'hui 
un  point  de  lumiere  tres  concentric  dans  son  milieu  [</<,].  Pres  de  1'endroit  ou  ce 
pont  touche  le  bord  boreal  du  Sinus,  mais  un  peu  suivant,  on  voit  distinctement  une 
baie  e*troite  qui  s'etend  dans  la  direction  du  lac  LASSELL.  Ce  lac,  de  forme  oblongue, 
s'&end  encore  conside'rablement  au  deh\  de  84  [663]  dans  la  direction  de  la  baie  indi- 
que*e.  Par  moments  il  m'a  paru  comme  si  la  baie  indiqu^e  et  le  lac  soient  r^unis  entre 
eux  par  un  canal  etrojt  qui  passe  pres  de  1'etoile  luisante  87  [669],  cependant  je  n'ai 
pas  obtenu  la  conviction  qu'il  n'y  existe  une  interruption  dans  ce  canal. 

1857,  Sept.  24.  La  baie  LAMONT  pent  etre  discerned,  mais  elle  est  remplie  de 
matiere  n^buleuse. 

1857,  Oct.  24.  Une  tache  noire  circulaire  [W1  =  locus  Secchii]  de  I5"de  diametre 
precede  70  [635]  un  peu  vers  le  sud.     Je  ne  me  rappelle  pas  1'avoir  note*  auparavant 
La  baie  LAMONT  parait  aujourd'hui  bien  sombre;  cependant  on  y  remarque  encore  des 
traces  de  matiere  n^buleuse. 

1857,  Oct.  27.  Le  lac  SECCHI  a  les  bords  tres-peu  definis,  mais  on  pent  le  recon 
naitre  encore  sans  difficult^.  Du  canal  entre  le  lac  LASSELL  et  le  Sinus  magnus  je 
crois  apercevoir  par  moments  rextre"mite  australe,  mais  pas  bien  distinctement.  La 
baie  LAMONT  est  remplie  de  matiere  ne*buleuse.  Un  canal  eVoit  joint  le  Shins  Gentilii 
avec  1'espace  obscur  boreal  [palus  Bondii].  Hier  je  ne  Tai  pu  reconnaitre,  mais 
aujourd'hui  je  le  vois  tres  distinctement. 

1859,  Feb.  28.  Le  lac  SECCHI  me  parait  aujourd'hui  beaucoup  moins  sombre 
qu'autrefois.  En  revanche  il  y  a  une  tache  tres  noire  pres  du  trapeze,  que  je  n'ai  pas 


70  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

remarquee  prece'demment.  Elle  suit  le  trapeze  un  peu  vers  le  sud  a  une  distance  de  pen 
de  secondes  de  la  ligne  D  C.  [This  is  the  space  bounded  nearly  by  636,  648,  628,  640, 
which  joins  the  canal  between  M  and  S.] 

1 86 1,  March  9.  Les  environs  du  trapeze  sont  remplis  de  matiere  ne'buleuse  tres 
condense'e,  de  sorte  qu'ils  ne  cedent  que  tres  peu  aux  plus  luisantes  parties  de  la  region 
Huyghenienne.  [I  presume  this  to  refer  particularly  to  the  east  side  of  A.  In  this  way 
it  agrees  best  with  present  appearances.] 

L'etoile  126  [793],  que  nous  voyons  placee  chez  HERSCHEL  un  peu  vers  le 
sud  de  la  pointe  du  promontoire  de  Proboscis  major  (Promontorium  Herschelii),  et  que 
BOND  a  dessine'e  sur  le  parallele  de  ce  promontoire,  a  la  distance  d'une  dizaine  de 
secondes,  se  trouve  aujourd'hui  decidement  sur  la  perite  bordale  du  promontoire,  exacte- 
ment  sur  la  limite  de  la  ne'bulosite.  Elle  suit  la  pointe  extreme  de  plusieurs  secondes. 

1 86 1,  March  27.  Le  pont  SCHROETER  joint  les  deux  bords  du  Sinus  magnus,  sans 
lumiere  plus  condense'e  au  milieu.  Le  canal  entre  le  lac  LASSELL  et  le  Sinus  magnus  pent 
etre  poursuivi  dans  toute  son  e'tendue ;  il  me  parait  maintenant  plus  large  que  pre'ce'- 
demment  En  revanche  le  canal  entre  le  Sinus  Gentilii  et  le  palus  Bondii  est  interrompu 
par  des  masses  ndbuleuses  assez  luisantes.  La  baie  LAMONT  peut  6tre  reconnue,  mais 
elle  est  remplie  de  matiere  ne'buleuse,  et  le  demi-cercle  luisant  qui  1'entoure  ordinaire- 
ment  [Hemicyclium-Liapunovii]  ne  se  voit  qu'a  peine.  Le  trapeze  se  trouve  au  milieu 
d'une  masse  ne'buleuse  tres  luisante ;  une  tache  presque  noire  touche  de  tres  pres  la 
ligne  C  D  en  s'titendant  j  usque  dans  le  voisinage  de  88  [671]. 

1 86 1,  Sept.  27.  La  baie  LAMONT  est  aujourd'hui  remplie  de  matiere  ne'buleuse 
tres  forte,  de  sorte  qu'on  la  reconnait  a  peine.  Uhemicycle-Liaponoff  a  presque  entiere- 
ment  disparu.  Le  pont  SCHROETER  joint  en  apparence  les  deux  bords  du  Sinus  magnus; 
a  1'inteYieur  du  pont  tout  1'espace  est  convert  d'une  masse  ne'buleuse  assez  intense,  de 
maniere  que  M.  WINNECKE,  qui  pour  la  premiere  fois  regardait  la  ne'buleuse  par  le 
grand  refnacteur,  croyait  d'abord  que  le  Sinus  magnus  ne  s'e'tendait  que  jusqu'au  pont. 
L'e'toile  no  [741]  touche  aujourd'hui  le  bord  du  Proboscis  major,  et  126  [793]  est 
de'cide'ment  sur  la  pente  bore'ale  du  promontoire  HerscheL 

1 86 1,  Dec.  2.  Le  palus  Bondii  est  aujourd'hui  tres  sombre,  mais  je  ne  puis  pas 
reconnaitre  le  canal  qui  se  joint  avec  le  Sinus  Gentilii.  Au  contraire  le  canal  entre  le 
lac  LASSELL  et  le  Sinus  magnus  parait  aujourd'hui  tres  large,  mais  en  meme  temps  moins 
sombre  que  d'ordinaire.  Le  lac  Secchi  a  les  contours  tres-mal  de'finis.  La  baie  Lamont 
est  aujourd'hui  moins  etendue,  mais  d'une  noirceur  assez  intense.  Le  sommet  du  pro- 
montoire Herschel  precede  I'e'toile  126  [793]  de  plusieurs  secondes,  ayant  en  meme 
temps  une  dticlinaison  plus  australe  de  20".  [This  is  so  in  Washington  observations, 
January  10,  1877,  for  example.]  Aussi  dans  les  parties  bore'ales  les  apparences  du 
Proboscis  major  different  aujourd'hui  essentiellement  de  celles  que  nous  offre  le  dessin 
de  Sir  J.  HERSCHEL.  Tout  le  Proboscis  parait  avoir  fait  un  mouvement  vers  1'ouest,  et 
le  promontoire  pres  de  l'(3toile  1 17  (778)  a  presqu'entierement  disparu.  En  tirant  une 
ligne  droite  par  les  deux  etoiles  Q"  et  no,  je  trouve  que  la  distance  de  0"  a  la 
limite  suivante  de  la  region  Huyghenienne  (ligne  A  B  de  M.  LIAPONOFF)  se  rapporte  a  la 
distance  de  1 10  au  Proboscis,  comme  i  a  1.5,  tandis  que  le  dessin  de  HERSCHEL  fait  ce 
rapport  au  moins  i  :  4. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION  7! 

Lepont  Schroeter  est,  dans  toute  son  etendue,  d'un  dclat  uniforme,  plutot  faible; 
mais  il  traverse  tout  le  Sinus  magnus.  Le  trapeze  est  plonge  dans  une  masse  ne'buleuse 
bien  intense ;  la  tache  sombre  observee  en  d'autres  occasions  du  cdte"  suivant  du  trapeze 
ne  peut  guere  etre  reconnu  aujourd'hui. 

1 86 1,  Dec.  29.  L'existence  de  75   [647]  fut  soupc,onnee  par  moments,  les  autres 
etoiles  ne  sont  pas  distinctement  reconnues.     Le  pont  Schroeter  joint  les  deux  bords  du 
Sinus  magnus,  sa  lumiere  est  consideYablement  plus  condensed  vers  le  milieu.     A  1'inte'- 
rieur  du  pont  [T~\  le  Sinus  est  parfaitement  noirdu  cote*  nord;  du  cote*  sud,  au  contraire, 
il  y  a  beaucoup  de  matiere  ne'buleuse.     [This  is  precisely  contrary  to  present  appear- 
ances.]    La  baie  Lamont  se  voit  avec  facilite*,  mais  elle  est  remplie  d'une  legere  matiere 
ne'buleuse;  a  son  embouchure  dans  le  Sinus  Gentilii,  il  y  a  aujourd'hui  un  fil  de  lumiere 
plus  intense,  qui,  en  forme  de  pont,  se'pare  les  deux  baies.     Ce  pont  n'a  jamais  e'te' 
apercu  par  moi  auparavant,  malgre"  toute  1'attention  que  j'ai  voue'e  en  toute  occasion  a 
cette  partie  de  la  ne'buleuse.     Le  trapeze  est  plonge  dans  une  masse  ne'buleuse  tres 
intense. 

1862,  March  6.  La  baie  Lamont  est  tres  sombre.     Le  pont  Schroeter  a  de  la  lumiere 
fortement  concentree  au  milieu,  mais  il  n'atteint  pas  le  bord  sud  du  Sinus. 

1862,  March  21.  La  baie  Lamont  s'est  re'tre'cie  en  apparence;  au  centre  elle  est 
bien  noire,  mais  les  bords  sont  converts  de  matiere  ne'buleuse ;  son  embouchure  dans 
le  Sinus  Gentilii  est  en  partie  ferme'e  par  un  fil  lumineux.  La  tache  noire  qui  suit  le 
trapeze  se  voit  tres  distinctement,  son  'e'tendue  me  parait  plus  petite  qu'autrefois. 

CONSIDERATIONS    SUR    LES    OBSERVATIONS    PRECEDENTES. 

Les  extraits  precedents  de  mon  journal  d'observations  contient  sans  doute  des 
indications  tres  fortes  de  changements  dans  I'e'tat  de  la  ne'buleuse.  Ne"anmoins  je  suis 
bien  loin  de  prdtendre  que  tous  les  changements  note's  soient  e'leve'  au  dessus  de  tout 
doute.  Au  contraire,  en  exei^ant  une  critique  seVere  il  n'y  reste  que  tres*peu  qui,  a 
mes  propres  yeux,  soit  bien  prouve*,  je  dirai  m£me  il  n'y  a  rien  de  prouve'  par  rapport 
a  des  changements  dans  la  ne'buleuse  elle-meme.  Les  deceptions  dans  ce  genre 
d'observations  sont  tellement  nombreuses  qu'on  ne  peut  pas  6tre  assez  sur  ses  gardes 
dans  ce  qu'on  avance  comme  des  faits  e^ablis.  MalgnS  la  bonne  volont^  de  se  tenir 
libre  de  toute  preoccupation,  Imagination,  supported  dans  ces  cas  par  1'insuffis.ance  de 
nos  moyens  d'observation  et  par  Teffet  de  1'etat  variable  de  1'atmosphere  nous  entraine 
facilement  a  voir  ce  que  nous  voulons  voir  ou  plutot  a  ce  qui  s'accorde  le  mieux  avec 
nos  pensees  intimes  et  d'un  autre  cote  a  ndgliger  de  noter  ce  qui  parait  s'opposer  i\ 

nos  vues. 

[p.  115.]  Passons  maintenant  aux  observations  concernant  la  distribution  et  I'&lat 
de  la  matiere  nebuleuse  elle-m^me.  fividemment  elles  n'accusent  presque  aucun 
changement  de  forme,  mais  bien  des  fluctuations  dans  1'^clat  des  differentes  parties. 
L'impression  generale  que  j'ai  gagn^e  par  ces  observations  est  que  la  partie  centrale  de 
la  nebuleuse  se  trouve  dans  un  etat  d'agitation  continuelle,  comme  la  surface  d'une  mer. 
Ici  je  me  bornerai  a  diriger  1'attention  des  astronomes  sur  les  particularite's  suivantes : 

0)  Sir  J.  HERSCHEL  a  place  le  trapeze  dans  un  espace  presque  vide  de  matiere 


72  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

nebuleuse.  En  general  je  suis  d'accord  avec  lui  sur  ce  point,  mais  il  y  a  eu  des  nuits, 
surtout  en  printemps  1861,  ou  la  nebulosite  en  dedans  du  trapeze  et  dans  son  voisinage 
immediat  m'a  paru  tout  aussi  forte,  que  dans  les  autoes  parties  les  plus  brillantes  de  la 
region  Huyghenienne.  Cela  me  parait  prouver  que  Pobserve  vacuum  n'est  pas  unique- 
ment  produit  par  Peffet  du  contraste.  Dans  des  nuits  ou  il  y  avait  plus  de  masse 
nebuleuse  en  dedans  du  trapeze,  j'ai  vu  a  differentes  reprises  une  tache  considerable- 
ment  plus  sombre  qui  s'etendait  a  Pest  du  trapeze  quelquefois  jusque  dans  les  environs 
de  Petoile  88  (671).  [This  is  the  canal  between  M  and  S.]  Cette  tache  n'est  indiquee 
chez  aucun  des  autres  observateurs.  HERSCHEL  et  BOND  indiquent  au  contraire  a 
1'endroit  de  cette  tache  une  condensation  plus  forte  de  la  matiere  nebuleuse. 

(b)  Sinus  Lamontii.  Sur  cette  baie  Pattention  a  ete  dirigee  deja  par  Sir  J.  HER- 
SCHEL qui  en  conteste  Pexistence.  (Cape  obs.,  p.  32.)  Elle  n'est  representee,  ni  dans 
son  dessin,  ni  dans  celui  de  BOND  [1848].  Mais  M.  LIAPONOFF  Pa  vu  de  tres  pres 
comme  elle  est  represented  par  M.  LAMONT.  Mes  observations  indiquent  que  tantot 
cette  baie  est  a  pen  pres  aussi  noire  que  le  Sinus  Gentilii,  tantot  qu'elle  est  remplie  de 
matiere  nebuleuse  presque  aussi  lumineuse  que  le  reste  de  la  region  Htiyglienienne. 

(d)  Le  pont  du  Sinus  magnus.  Le  dessin  de  HERSCHEL  indique  a  1'endroit  de  ce 
point  un  promontoire  assez  faible.  Plus  tard  M.  LIAPONOFF  orne  ce  promontoire  d'une 
pointe  brillante  [<70]  mais  aussi  cet  astronome  ne  le  fait  s'etendre  que  jusqu'au  milieu 
du  Sinus.  Les  dessins  de  BOND  et  de  M.  LASSELL  n'offrent  pas  de  traces  de  cette 
formation.  Au  contraire  il  y  a  meme  chez  BOND  une  tache  plus  sombre  a  Pendroit  ou 
M.  LIAPONOFF  place  la  pointe. 

Mes  propres  observations  etendent  le  promontoire  presque  toujours  jusque  dans 
le  voisinage  du  bord  sud  du  Sinus  et  quelquefois  m6ine  il  m'a  paru  qu'il  n'y  avait  plus 
aucun  intervalle,  circonstance  qui  lui  a  valu  la  designation  de  pont,  Au  milieu  de  ce 
pont  j'ai  remarque  presque  toujours  un  point  plus  lumineux,  qui  s'accorderait  avec  la 
pointe  indiquee  par  M.  LIAPONOFF,  mais  il  y  en  aussi  des  jours  oil  le  pont  m'a  paru  de 
lumiere  uniforme  dans  toute  son  etendue.  [#0  was  first  plainly  seen  by  SCHROETER. 
Since  1800  it  has  been  seen  on  many  occasions,  and  again  under  equally  good  condi- 
tions its  absence  has  been  noted.  The  Washington  observations  are  remarkable  in 
this  respect,  and  agree  in  general  with  the  conclusions  of  STRUVE.] 

(/)  La  masse  nebuleuse  situee  au  nord  de  Petoile  75  (647)  [D].  II  m'a  paru  que 
cette  masse  fut  sujette  a  de  variations  bien  considerables  tant  en  dimensions  qu'en 
intensite.  [The  same  appearances  have  been  remarked  at  Washington.] 

(</)  Le  promontoire  Herschel  sur  le  Proboscis  major.  Dans  son  dessin  de  1837,  Sir 
JOHN  HERSCHEL  place  Petoile  126  (793)  sur  la  pente  australe  du  promontoire,  en  con- 
tact apparent  avec  la  matiere  nebuleuse.  En  1847  elle  s'est  trouve'e,  d'apres  BOND,  au 
dessus  de  la  cime,  mais  separee  d'elle  par  un  espace  obscur.  A  Pepoque  actuelle  il 
n'y  a  pas  de  doute  qu'elle  se  trouve  de  nouveau  en  contact  avec  la  matiere  nebuleuse, 
mais  deja  considerablement  siir  la  pente  bor&de.  N'y  aurait  il  pas  ici  un  indice  d'un 
changement  progressif  dans  la  configuration  de  la  "nebuleuse '?  Au  moins  il  est  prouvd 
que  Petoile  n'a  pas  change  sensiblement  sa  position  dans  Pintervalle  de  1837  ^  I^5°> 
et  il  parait  inadmissible  de  supposer  que  HERSCHEL  ait  pu  se  tromper  d'autant  des  les 
positions  respectives  de  la  cime  et  de  Petoile,  qu'il  ait  place  cette  derniere  sur  la  pente 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  73 

australe,  si,  comme  aujourd'hui,  elle  s'dtait  Mfi  trouvee  en  1837  du  cot<<  boreal.  [The 
position  given  by  STRUVE  for  this  star  agrees  with  my  own.] 

(h)  Le  coin  boreal  du  Proboscis  major.  Dans  la  derniere  annee  toute  cette  partie 
parait  avoir  fait  un  mouvement  vers  le  sud-ouest.  Sans  la  supposition  d'un  change- 
ment  il  serait  impossible  de  concilier,  sur  ce  point,  mes  dernieres  observations,  avec 
les  dessins  des  autres  astronomes  et  m6me  avec  mes  propres  observations  de  1857." 

The  preceding  observations  and  remarks  have  been  given  at  some  length  both 
on  account  of  their  importance  and  because  they  contain  a  careful  arid  detailed  com- 
parison of  the  works  of  LAMONT  and  HERSCHEL.  LIAPONOFF  was  the  first  to  carry  out 
the  idea  suggested  by  SCHROETER  of  making  a  minute  study  of  single  parts  of  a 
nebula,  and  it  appears  to  me  that  the  faithful  and  accurate  manner  in  which  this 
critique  was  done  has  not  received  all  the  commendation  it  deserves.  In  this  way, 
also,  we  may  secure  the  advantage  of  the  minute  criticism  which  Dr.  OTTO  v.  STRUVE 
has  given  to  preceding  memoirs. 

OBSERVATIONS  OF  LASSELL  (1854). 

I  extract  from  LASSELL'S  account  of  his  observations  on  the  nebula  of  Orion,  Mem. 
R.  A.  S.,  xxiii,  p.  53,  et  seq.,  such  notes  as  appear  to  throw  the  most  light  upon  his  views 
concerning  it.  These  observations  were  taken  with  his  admirable  reflector  of  2  feet 
aperture,  at  Malta.  "Wednesday,  Dec.  15,  [1852].  *  *  *  With  101 8,  the  wool- 
like  masses  appear  as  I  have  previously  described  them,  and  there  is  no  disposition 
whatever  in  them  to  turn  into  stars." 

"  7th  Jan.,  1853.  The  nebula  of  Orion  was  surveyed  under  better  circumstances 
than  yesterday,  without  my  seeing  anything  remarkably  different  from  former  observa- 
tions. The  pea-green  color  of  the 
nebula  is  very  remarkable — differ- 
ent in  this  respect  from  all  others — 
as,  indeed,  it  is,  I  think,  the  bright- 
est of  all  the  nebulae  I  have  seen. 
*  *  *  I  tried  several  higher 
powers  *  *  *  on  the  brightest 
parts  of  the  nebula,  but  they  only 
strengthen  my  former  impressions 
of  my  inability  to  resolve  it  with 
my  utmost  means." 

"In  order  to  perpetuate  as  far 
as  possible  the  results  of  these 
observations,  I  send,  herewith,  a 
painting,  in  oil,  of  this  nebula  on 
the  same  scale  as  my  original  draw- 
ings. *  *  *  It  is  the  work  of 
my  friend,  Mr.  HIPPISLEY,  executed 
under  my  own  superintendence 

f  -it  i        «,i  FIG.  27.     LASSEI.L  ,  1854. 

and  carefully  compared  with  my 
APP.  V 10 


74 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


original  sketches.  I  consider  it  a  very  faithful  picture  of  what  I  saw,  when  placed  in 
a  proper  light  and  well  illuminated.  Without  attention  to  these  conditions  the  nebula 
will  appear  too  faint.  *  *  *  I  send  also,  herewith,  a  similar  drawing  on  a  smaller 
scale.  *  *  *  I  have  endeavored  in  these  drawings  to  represent  the  original  as 
closely  as  possible,  comparing  and  improving  my  original  sketches  night  after  night 
with  the  nebula  itself."  [This  second  drawing  is  given  as  Plate  I  of  the  volume  already 
cited,  and  is  reproduced  as  figure  27  of  this  text].  In  the  same  volume  of  the  Memoirs 
a  portion  of  a  private  letter  of  LASSELL'S  is  quoted  (p.  108),  in  which  he  says:  "A 
comparison  of  Sir  JOHN  HERSCHEL'S,  Mr.  BOND'S*,  and  my  own  drawings  of  [the 
nebula  of  Orion]  must,  I  think,  suggest  the  idea  of  change  in  the  nebula  or  varia- 
bility of  the  stars,  or  otherwise  a  less  uniformity  of  delineation  of  the  same  thing 
than  might  have  been  hoped  for." 

In  this  figure  attention  should  be  directed  to  the  brightness,  or  want  of  brightness, 
of  the  parts  just  preceding  the  trapezium  and  of  the  n.  f.  parts  of  the  Huyyhenian  region. 
I  add  (from  MS.  by  G.  P.  BOND  put  at  my  disposition  by  the  Harvard  College 
Observatory)  a  list  of  identifications  of  the  small  stars  of  LASS  ELL'S  chart. 

Small  stars  in  LASSELL'S  chart  (Mem.  R.  A.  S.,  xxiii).  [The  stars  on  this  chart  are 
G.  P.  B.  567,  575,  589,  595  (c),  608  (/),  612  (t),  618  (/*),  621  (c),  622,  625  (d),  636, 
647  (/),  648,  651,  671,  and  676  (&),  #!!,  675  (a),  685,  H?,  708,  741,  besides  the  6 
stars  of  the  trapezium.  BOND  has  no  stars  corresponding  to  g  and  b;  601,  602,  642, 
654,  686,  688  of  BOND'S  list  are  within  the  limits  of  LASSELL'S  chart  and  not  mapped 
by  him. 

OBSEEVATIONS  OF  SCHMIDT  (1860-75). 

The  observations 
made  by  Dr.  J.  F.  W. 
SCHMIDT,  director  of  the 
Observatory  of  Athens, 
have  been  embodied  in 
two  drawings  by  him 
which  he  has  most  kindly 
communicated  to  me  and 
put  at  my  disposition. 
The  first  and  most  elabo- 
rate of  these  is  given  here- 
with (Fig.  28)  The  other 
relates  to  the  nebula  as 
a  whole,  and  is  preserved 
for  reference.  This  work 
has  not  yet  been  pub- 
lished and  no  description 
has  yet  been  given.  For- 
tunately the  drawing  is 
complete  in  itselt. 

The  different  masses  are  well  shown,  and  the  amount  of  detail  is  quite  striking 
for  the  6-inch  refractor  used  by  Dr.  SCHMIDT. 


FIG.  28.     SCHMIDT,  1861. 


.    MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  -r 

/  D 

OBSERVATIONS  OP  SECCHI  (1862). 

The  drawing  of  SECCHT,  1862,  given  in  the  Astronomlsche  Nachrichten,  Band  xlv, 
col.  60,  refers  more  particularly  to  the  nebula  as  a  whole,  and  is  reproduced  in  Fig.  29. 


FIG.  2.9.      SECCHI,  1862. 


It  is  used  here  simply  to  give  the  general  form  of  the  whole  nebula.     It  has  been 
severely  criticized  by  D'ARREST.     The  drawing  is  inverted. 


-6  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.   • 

OBSERVATIONS  OF  TEMPEL  (1862). 

The  early  drawing-  by  TEMPEL,*  given  in  Fig.  30,  is  also  introduced  for  a  com- 
parison with  the  drawings  of  the  early  observers,  and  being  about  contemporaneous 
with  that  of  SECCHI  may  also  be  compared  with  it. 


FIG.  30.     TEMPEL,  1862. 
OBSERVATIONS  OF  LASSELL  (1862). 

LASSELL'S  drawing1  of  1862  was  made  by  Miss  CAROLINE  LASSELL,  an  accom- 
plished artist,  and  one  familiar  with  astronomical  work  of  this  kind,  by  means  of  the 
4-foot  equatorial  of  Mr.  LASSELL'S  construction  during  his  celebrated  astronomical 
expedition  to  Valetta.  It  was  not  published  with  the  admirable  collection  of  drawings 
made  at  the  same  timef  on  account  of  its  large  size.  The  scale  was  100"  =  1.194 
inches. 


From  Astronomische  Nachrlchten,  vol.  Iviii,  col.  240.  t  Memoirs  Royal  Astronomical  Society,  vol.  36. 


L  (1862). 

' 


accom- 

of  the 
tronomical 

coli  I'  drawings 

>o"  —  1.194 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  77 

A  tracing  of  this  was  made  by  Mr.  LASSELL  himself  in  1876,  and  after  I  had  had 
an  opportunity  of  inspecting  the  original  at  Ray  Lodge,  Maidenhead,  the  artist  kindly 
offered  to  make  a  fac-simile  of  it,  which  is  now  in  my  possession.  A  photograph  o'f 
the  central  portions  only  is  reproduced  in  Fig.  31,  and  I  cannot  but  regret  that  the 
drawing,  as  a  whole,  is  not  generally  accessible,  embodying  as  it  does  the  results  of 
great  labor  and  skill  employed  under  most  favorable  circumstances. 

It  is  to  be  remarked,  that  in  this  drawing  there  re-appears  for  the  first  time  since 
SCHKOETER'S  1798  drawings  (with  the  possible  exception  of  HERSCHEL,  1826),  the 
second  bridge  of  SCHROETEE.  It  is  there  plainly  laid  down,  as  it  was  subsequently 
seen  by  Lord  ROSSE  and  by  myself,  and  much  as  described  by  SCHROETER.  In  the 
subsequent  discussion  (Part  III)  I  shall  recur  to  this  feature.  A  strong  suspicion  of  its 
variability  in  brightness  is  excited  as  it  was  seen  by  SCHROETER  with  a  reflecting  tel- 
escope of  19  inches  aperture,  and  remained  unseen  until  LASS  ELL'S  work,  and  was 
always  unknown  even  to  the  indefatigable  BOND,  who  observed  the  nebula  under  most 
favorable  circumstances. 

OBSEEVATIONS  OF  STEUVE  (1863). 

OTTO  STRUVE  in  Melanges  Mathematiques  et  Astronomiques,  iii,  p.  539,  thus  describes 
his  examination  of  the  Otow-nebula  with  the  4-foot  telescope  of  LASSELL,  which  was  at 
this  time  (1863)  mounted  at  Malta.  Comparison  observations  were  made  at  the  same 
time  by  Dr.  WINNECKE  at  Pulkova.  It  will  be  remembered  that  the  joint  work  of 
STRUVE  and  LIAPONOFF  on  this  nebula  was  published  in  1862,  and  that,  therefore,  all 
the  details  must  have  been  fresh  in  STRUVE'S  memory.  His  report  is,  therefore,  of  high 
importance:  "  Es  ging  jedoch  meine  Erwartung,  hier  an  demselben  allerhand  neue  For- 
men  und  Details  zu  sehen,  die  ich  in  Pulkowa  nicht  erkenrit  hatte,  nichtin  Erfiillung: 
wie  denn  iiberhaupt  der  gesammt  Eindruck  des  Bildes,  der  Charakter  der  Erscheinung, 
hier  genau  derjenige  war,  mit  dem  ich  durch  die  vielfachen  Beobachtungen  an  un- 
serem  Refraktor  vertraut  geworden  bin,  nur  etwa  mit  dem  Unterschiede,  dass  hier 
eirizelne  Theile  etwas  heller  erschienen  und  dadurch  bestimmter  hervortraten  als  wie 
bei  uns."  Neither  STRUVE  nor  LASSELL  under  favorable  circumstances  saw  any  indi- 
cations of  the  resolvability  of  this  nebula,  and  in  the  Huyghenian  region  STRUVE 
found  only  five  stars  which  he  had  not  seen  there  in  Pulkova. 

"In  Bezug  auf  Veranderungen  in  der  Nebelmaterie  boten  die  beiden  Nachten  in 
Malta  nur  eine  einzige  aber  sehr  entschieden  dastehende  Thatsache.  Am  8.  Oct. 
erstreckte  sich  namlich  pons  Schoeteri  kaum  bis  auf  die  Halfte  des  Sinus  magnus  und 
zeigte  nirgends  mehr  concentrirtes  Lichte,  wahrend  er  am  10.  Oct.  bis  nahe  an  die 
stidliche  Begranzung  der  Bucht  reichte  und  die  gewohnliche  Verdichtung  von  Nebel- 
materie nahezu  auf  der  mitte  deutlich  hervortrat.  In  dieser  Wahrnehmung  stimnite 
Herrn  MARTH'S  Urtheil  vollkommen  mit  dem  meinigen  uberein."  STRUVE  remarks 
that  if  either  of  the  nights  (8th  and  loth  October,  1863),  which  he  elsewhere  speaks 
of  as  "herrlichen"  was  better  than  the  other,  that  of  the  8th  was  to  be  preferred. 

The  description  indicates  a  difference  in  position  of  the  bridge  of  about  thirty 
seconds  of  arc,  and  it  is  not  at  all  probable  that  any  real  variation  in  shape  could  be 
remarked  in  48  hours ;  so  that  the  difference  in  aspect  must  be  ascribed  to  something- 
inside  the  earth's  atmosphere. 


78  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

"Der  Stern  126,  iiber  dessen  success! v  veranderter  Stellung  zum  Promentorium 
Herschelianum  mein  Memoire  Andeutungen  enthalt,  stand  jetzt  erheblich  auf  dem  nord- 
lichen  Abhange  jenes  Vorgebirges,  und  war  urn  mehrere  Secunden  von  dichter  Nebel- 
materie  iiberragt.  Hierin  scheint  sich  ein  continuirliches  Fortschreiten  der  angedeu- 
teten  Veranderungen  in  diesem  Theile  der  Proboscis  major  auszusprechen." 

With  regard  to  the  spiral  nature  of  this  nebula,  which  was  pointed  out  by  G.  P. 
BOND,  both  LASSELL  and  STRUVE  agree  in  certifying  to  the  reality  of  the  curved  lines 
which  BOND  described.  Their  attention  was  particularly  directed  to  the  curve  of  light 
which  starting  fro'm  the  preceding  side  of  the  regio  Huygheniana  stretches  far  into  the 
reglo  Gentiliana.  STRUVE  is  certain  that  this  was  not  quite  continuous  throughout  its 
length,  and  that  it  is  broken  in  the  narrow  canal  which  joins  the  palus  Bondii  with  the 
Sinus  Gentilii. 

OBSERVATIONS  OF  WEBB  (1863-1876). 

By  the  kindness  of  the  Rev.  T.  W.  WEBB,  I  have  access  to  two  original  pencil- 
drawings  by  himself,  as  well  as  MS.  notes  of  his  observations.  Such  of  the  notes  as 
refer  to  the  central  parts  I  give  below,  exactly  as  they  were  communicated,  and 
beside  these  I  have  given  in  Part  III  inferences  derived  from  the  original  drawings. 

The  first  drawing  was  begun  1863,  Dec.  29,  and  was  continued  on  the  following 
dates:  1864,  Jan.  i,  4;  Feb.  9,  12;  March  4,  Dec.  27,  and  Dec.  30.  On  the  last  two 
dates  the  following  notes  are  written  on  the  drawing:  "Dec.  27.  I  think  there  is  a 
dark  channel,"  [connecting  Sinus  Gentilii  with  dark  spaces  toward  the  n  p  ]  "  The 
light  n.  p.  the  trapezium"  is  connected  by  a  line  which  indicates  the  p.  edges  of  J  and 
B  as  they  are  at  present.  On  Dec.  30  this  dark  channel  was  again  seen. 

The  drawing  of  1866  was  made  Feb.  17,  and  has  this  note:  "The  three  cumuli 
[F,  G,  H]  nearest  93,  [685  G.  P.  B.]  form  an  equilateral  triangle."  H  is  marked 
"very  feeble." 

The  following  observations  on  the  great  nebula  in  Orion  were  made  by  T.  W. 
W^BB  with  a  5-in.  objective,  by  ALVAN  CLARK: 

1863,  Oct.  5d.  Rather  flaring  definition.  I  did  not  know  exactly  where  to  look 
for  D'ARREST'S  "bridging  over"  of  the  Sinus  magnus,  and  definition  was  fluttering,  and 
with  considerable  moonlight;  yet  164  showed  a  nebulous  bridge  very  feeble,  bat  quite 
certain,  in  the  line  between  2  stars  marked  in  the  diagram  [669  and  685]  ;  now  arid 
then  I  fancied  that  a  minute  star  peeped  through  about  the  middle  of  the  bridge  [</0]  ; 
the  space  inside  it  [r]  (in  BOND'S  drawing,  the  darkest  in  the  Sinus]  was  less  dark,  as 
though  veiled  over ;  216  showed  the  same,  but  without  the  star ;  64  did  not  make  it 
out  well ;  460  showed  it  faintly,  but  not  the  interior  veil.  With  64  and  164  it  could 
not  be  doubted.  [The  pons  Schroeteri  of  0.  STRUVE.] 

1863,  Nov.  14*.  Very  low,  and  definition  frightful;  no  hope  of  dividing  <?.  Yet 
pons  Schroeteri  visible  with  164  and  205  ±,  and  inner  space  [r]  considerably  brighter, 
especially  by  averted  vision.  With  164,  I  fancy  something  like  a  bifurcation  of  the 
end  of  the  N.  side  of  the  Sinus  magnus. 

1863,  Dec.  29d.  Fine,  but  a  little  fluttering.  Pons  Schroeteri  plain  ;  no  star  in  it; 
(power  seems  to  have  been  204  i  and  440).  5th  star  in  trapezio  difficult ;  6th  not 
seen. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  79 

1865,  Jan.  2od.  Very  transparent  night,  but  much  flare.  Nebulosity  within  pans 
Schroeteri  \_r]  very  evident,  65.  Later  this  was  less  plain,  but  the  whole  Sinus  magnus 
seemed  slightly  nebulous.  [£  and  T'~\. 

1865,  Jan.  2id.  Very  cold  night;  very  fine  definition.     No.  6  certainly  seen  at 
times  with  204  ±,  but  not  with  45 1  or  a  microscopic  triplet.     But  I  never  yet  saw  the 
trapezium  so  beautifully. 

1866,  Jan.  5d.  Bad  and  fluttering  definition.     About  8h,  when  not  more  than  half 
way  to  the  meridian,  I  saw  with  1 1 1  very  evident  traces  of  the  neb.  oblongata,  which, 
as  far  as  my  recollection  will  serve,  I  have  in  former  seasons  several  times  looked  for 
in  vain,  under  better  circumstances.     It  was  now  faint,  but  quite  certain      The  pans 
Schroeteri  was  very  evident.     At  a  later  hour  a  dark  irregular  rift  was  noticed  with  1 1 1 
and  212  between  the  2  stars  87  [635]  and  70  [669]  and  the  trapezium,  but  nearer  to 
the  former.     It  probably  communicated  with  the  Sin.  mag.  at  the  E.  end,  and  extended 
a  considerable  distance  p,  forming  a  dark  spot  [W3]  which  stood  nearly  equilaterally 
with  67  and  70      This  was  the  darkest  portion  of  it.     Hence  it  stretched  to  the  E.  in 
nearly  a  straight  line  of  irregular  breadth,  being  wider  W.  of  87  than  W.  of  70  [135]. 
Halfway  between   65   [619]  and  87  [669],  but  a  very  little  P.  the  line  is  a  bright 
knob,  at  times  seeming  to  inclose  a  star.     [S.  point  of  D I  with  647  !] 

[The  "  dark  spot"  is  probably  part  of  lacus  Lassellii  with  Wlt  W3,  and  W5.  I 
judge  this  is  so  from  a  diagram.] 

1866,  Jan.  nd.  Rather  fuzzy  definition,  in  (power).  Though  near  the  meri- 
dian, I  did  not  see  my  dark  rift  so  well  as  on  Jan.  5d.  I  could,  however,  distinctly  make 
out  the  dark  opening  on  lake  to  which  it  leads  [W2],  and  noticed  that  in  the  triangle 
it  forms  with  65  [619]  and  70  [635]  the  side  from  the  lake  to  70  seemed  rather  shorter 
than  the  other  two  The  N.  edge  of  the  cleft  passing  87  and  70  seems  a  continuation 
of  the  N.  edge  of  the  Sinus  magnus,  the  rift  extends  probably  right  through  the  more 
luminous  region  of  the  nebula.  I  do  not  think  the  projecting  end  of  the  reg.  Huyg  [E]  ? 
quite  so  conspicuous,  as  compared  with  the  S  P  and  S  F  masses  as  when  I  sketched 
it  in  1863  and  1864.  The  neb.  obi.  is  faintly  but  decidedly  visible. 

1866,  Jan.  25d.  Small  disks  behind  a  great  undulating  flutter.  The  rift  may  still 
be  detected,  especially  by  averted  vision,  with  in,  notwithstanding  a  moon  2  days 
past  ist  qu.  It  seems  to  be  feebly  traceable  beyond  the  lake,  as  a  N  P  border  to  the 
brightest  part  of  the  reg.  Huyg.  running  in  fact  from  the  S.  mag.  straight  across  to  a 
large  and  conspicuous  dark  opening,  not  distinctly  shown  in  any  drawing  which  I 
have,  except  that  by  BIRD,  but  forming  a  continuation  of  the  direction  of  the  Sin. 
Gent,  with  which  it  is  connected  by  a  long,  narrow  channel,  discovered  by  me,  1864, 
Dec.  27,  and  confirmed  on  Dec.  30.  Under  these  dates  I  have  no  marginal  memo- 
randum on  my  sketch  as  to  the  great  lake  into  which  this  channel  leads,  but  it  was 
rather  beyond  my  sketch  at  that  time.  I  have  of  late  noticed  it  more  than  once  as  a 
conspicuous  long  dark  opening,  which  now  seems  to  me,  with  the  Sin.  Gent.,  the  con- 
necting channel,  and  my  new  rift  to  insulate  the  brightest  part  of  the  nebula  on  two 
sides  from  the  adjacent  nebulosity.  In  the  triangle  67,  70,  lake  (see  Jan.  5d),  70  lake 
was  certainly  shorter  than  the  other  sides.  With  450  the  rift  is  still  pretty  distinct,  the 
triangle  is  neither  equilateral  nor  isosceles,  the  F  side  being  longest,  P  next,  and  N 
shortest.  The  space  included  by  the  4  lobes  of  light  S.  of  the  trapez.  [E,  F,  G,  I] 
is  comparatively,  I  fancy,  darker  than  formerly. 


8o  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1866,  Feb.  I7d.  Very  small  disks,  but  twirling  too  much,   212,   450.     5th  star 
very  plain  ;  no  6th.     76,  84,  89  visible.     I  cannot  see  80 ;  pons  Sclirot,  and  included 
nebulosity  very  evident.     I  do  not  see  neb.  oblong.     N.  end  of  Sin.  mag.  projects  about 
as  far  as  end  of  W.  edge  of  S.  side.     It  is  clearly  bifid.     I  do  not  see  the  canal  very 
plainly,  but  the  lake  is  pretty  visible;  beyond  "it  is  a  stretched  out  mass  of  nebulosity. 
450  shows  5  masses  in  frons  and  occiput;  the  3d  [H]  reckoned  along  the  frons  does 
not  reach  93  [685]  which  is  surrounded  with  strong  nebulosity.     The  end  of  S.  side 
[qu.  NJ]  of  Sin.  mag.  has  a  finger,  and  a  rounded  projection  N.  of  it.     I  doubt  the 
continuation  of  my  canal  through  the  lake.     Later,  I  see  neb.  obi.  and  my  rift  better 
with  450,  the  frons  appears  to  consist  of  6  cumuli,  of  which  the  first  3  are  more  dis- 
tinct and  less  run  together  than  the  last.     The  bottom  of  the  Sin.  Gent,  seems  to  fall 
just  against  the  opening  between  the  last  great  cumulus  of  the  occip.,  and  a  large,  less 
distinct  round  mass  IS1P  [between  I  and  J].     Cumuli  2  and  3  of  frons  form  an  equi- 
lateral triangle  with  one  in  the  interior.     [F,  H,  and  G.] 

The  following  observations  were  made  with  a  silver-on-glass  speculum  by  WITH., 
9.38-in.  aperture: 

1867,  Feb.  2d.  Blotty  air;  power,  212.     No.  5,  reddish;  6  not  distinct  in  bad  air. 
Blackness  of  Sin.  mag.  very  striking  just  outside  [following  I]  the  pons  Schroteri. 

1867,  Mar.  2d.  Bad  definition.  1 1 1  4-.  The  dark  rift  and  Jake  of  last  spring  are 
pretty  well  seen. 

1869,  Nov.  9d.  A  fine  night.  65.  4  in  trap,  very  obvious,  though  so  low;  and 
spiral  character  of  wisp  round  108  [734]. 

1869,  Dec.  id.  Definition  especially  bad,  the  focus  showing  the  disturbance-plane 
to  lie  very  near  the  earth.  There  is  not  only  a  strong  nebulosity  round  /,  with  65, 
notwithstanding  the  state  of  the  air,  but  two  considerable  stars  further  S.  are  evidently 
involved  in  a  similar,  but  separate,  mass  of  it.  With  450  I  thought  there  were  strong 
indications  of  my  rift  and  lake. 

1869,  Dec.  i  id.  Good  definition,  but  Orion  too  low.  450,  5th  in  trap.,  which  had 
been  feebly  traced  with  1 10,  could  be  well  made  out,  but  would  hardly  have  been 
discovered.  The  knob  of  haze  at  the  end  of  the  Sin.  mag.  is  very  conspicuous.  In 
the  direction  of  the  longest  diagonal  in  trapez.  is  a  luminous  ray,  at  3  or  4  times  the 
distance  of  the  2  stars,  between  which  and  the  trapez.  the  nebulous  films  in  a  trans- 
verse direction.  The  nebula  in  this  region  is  strikingly  pulled  out,  as  it  were,  into 
lengthened  streaks,  lying  in  various  directions.  [A!] 

1869,  Dec.  28d.  Definition  not  good.     With  450  I  make  out  pretty  fairly  the  dark 
rift  and  lake  formerly  observed.     See  1866,  Jan   5d. 

1870,  Jan.  25d.   Unusually  good  definition  (no  powers  specified).     5th  distinct, 
but  not  bright  or  even  obvious ;  it  might  easily  have  passed  unnoticed.     6th  I  could 
not  see.     The  interior  of  the    trapezium,  though   fainter   than    the   regio  Huyg.,  is 
decidedly  and  strongly  nebulous,  as  compared  with  the  Sin.  magnus. 

1874,  Feb.  I9d.  Fluttering  definition,  but  clear  air.  5th  in  trapez.  examined  for 
HUGGINS  with  BROWNING'JS  E  eye-piece  zz  dr  357-  I  glimpsed  it  occasionally  with 
difficulty,  but  for  the  most  part  it  was  invisible.  I  have  remarked  the  same  with  450 
on  more  than  one  occasion  during  the  present  season. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  gl 

1874,  Mar.  i8d.  Thin  haze.  5th  in  trap  ;  occasionally  seen  with  ±357,  but  never 
would  have  been  discovered.  • 

OBSERVATIONS  OF  BIED  (1866). 

A  drawing  by  FREDERICK  BIRD,  Esq.,  of  England,  bearing  the  date  of  1866,  Jan- 
uary, has  been  also  courteously  communicated  by  the  Rev.  T.  W.  WEBB.  It  was 
made  by  means  of  a  1 2-inch  silvered  glass  reflector.  No  notes  accompany  the  original. 
The  masses  A,  B,  etc.,  are  not  separately  laid  down,  and  the  following  notes  which  I 
have  deduced  from  the  drawing  are  somewhat  uncertain  from  this  cause. 

Decidedly  the  brightest  portion  of  the  Huyghenian  region  is  that  including  the 
masses  F  and  G.  This  portion  is  brighter  than  A  or  D.  The  apex  of  this  region  (E) 
is  fainter  than  G,  H,  etc.,  and  appears  to  be  about  equal  to  B. 

The  peculiarities  of  figure  I  have  not  specially  examined,  as  we  have  contemporary 
drawings  with  larger  telescopes. 

OBSERVATIONS  OF  G.  P.  BOND  (1859-1865). 

Through  the  kindness  of  Prof.  E.  C.  PICKERING,  director  of  Harvard  College 
Observatory,  I  was,  in  August,  1877,  allowed  access  to  the  original  papers  of  my  cousin, 


FIG.  32.     G.  P.  BOND,  1865. 


APP.  V— 11 


82  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

GEORGE  BOND,  at  the  Observatory  in  Cambridge,  which  I  visited  for  the  purpose.  The 
following  notes  referring  to  the  central  regions  of  the  nebula  of  Orion  are  given  in 
addition  to  those  selected  for  publication  in  the  m  Annals  of  this  Observatory,  vol.  v, 
p.  155  et  seq,  and  these  are  given  in  the  words  of  the  writer  with  such  explanations  as 
seem  to  be  required. 

The  exquisite  steel  engraving  made  under  BOND'S  direction  is  reproduced  in  the 
frontispiece  by  the  kind  permission  of  the  director  of  the  Harvard  College  Observatory, 
who  has  lent  the  original  steel  plate.  Fig.  32  gives  its  central  parts  on  an  enlarged 
scale.  The  frontispiece  is,  to  my  eye,  the  most  satisfactory  representation  of  any 
celestial  object  which  has  yet  been  produced.  It  was  corrected  and  revis  ed  many  times, 
and  the  artist  (J.  W.  WATTS,  Esq.)  had  the  use  of  the  Harvard  College  refractor  for 
several  years  in  order  to  study  the  pictorial  effect.  The  MS.  drawings  of  BOND  abund- 
antly show,  however,  that  the  forms  of  the  masses,  etc.,  are  due  to  him. 

EXTRACTS  FROM  OBSERVING-BOOKS  OF  HARVARD  COLLEGE  OBSERVATORY. 

[Also,  see  BOND'S  Memoir,  pp.  155  et  aeq.] 

G.  P.  BOND,  observer.    Notes  in  square  brackets  are  by  myself. 

Frons. 

1859,  Feb.  23.  The  edge  [frons  and  south  shore  of  Sinus  magnus]  seems  to  be  just 
perceptibly  brighter  than  the  [Huyglieniari]  region  within  it.  [A  sketch,  omitted  here, 
gives  a  strip  "as  wide  as  the  trapezium"  along  the  frons,  which  is  presumably  the 
brighter  strip.] 

A. 

[A]  is  the  brightest  part  of  the  nebula.  [Its  following  outline  laid  down.]  This 
is  verified  1859,  March  4. 

Sinus  Gentilii 
is  very  black. 

D. 

[D]  is  bright. 

1859,  March  23.  [As  twilight  came  on  the  relative  brightness  of  the  various 
masses  was  noticed  by  observing  the  order  of  their  appearance.  The  first  mass  to 
appear  was  A,  and  the  second  was  H  —  c0  of  LiApeNOFF.] 

1862.  March  27.  The  smallest  stars  in  the  bright  masses  of  nebulosity  about  the 
trapezium  are  easily  seen  in  strong  twilight  and  before  others  in  darker  regions  come 
in  sight,  although  when  the  sky  becomes  dark  the  latter  are  much  more  easily  seen. 

This  shows  that  the  small  stars  near  the  trapezium  are  really  much  brighter  than 
they  appear  to  be,  their  light  being  commonly  overpowered  by  that  of  the  nebula. 
This  fact  is  important  as  evidence  of  a  clustering  of  stars  about  the  brighter  nebulous 


regions. 


1863,  Jan.  1 8.  [The  direction  of  the  following  side  of  pons  Schroeteri  is  as  in  figure 
(omitted).]  The  line  limiting  it  passes  through  669  and  a  point  #  of  the  distance 
from  685  to  708  measured  from  685. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  83 

1863,  Jan.  19.  [Following  edge  of  pom  Schroeteri  is  along  a  line  joining  685  with 
No.  669  ?  ;  g0  seen.] 

1863,  Jan.  30.  [F]  is  equilateral. 

1863,  Feb.  23.  "The  drawing  of  1859  in  detail  was  compared  with  nebula  in  the 
end  of  February,  1863,  and  no  change  of  any  prominent  feature  could  be  recognized." 

[From  sketches  it  appears  that  the  dark  channel  between  F  and  G  is  in  the  pro- 
longation of  685,  708.  A  is  plainly  laid  down.  647  and  651  both  inside  the  border..] 

1863,  Dec.  7.  I  always  look  at  Sinus  magnus,  etc.,  for  change  of  feature,  but  was 
never  satisfied  of  any  not  accounted  for  by  change  of  atmospheric  conditions. 

1864,  March  19.  "In  very  early  twilight  the  bright  [Huyghenian]  region  is  defined 
so  as  to  extend  the  borders  of  Sinus  magnus  to  the  preceding  side  of  #'." 

[This  is  of  importance  in  connection  with  the  same  appearances  laid  down  by 
PICAKD  and  LE  GENTIL  and  in  my  own  drawings  made  through  tourmaline  plates.] 

Note  error  of  HEKSCHEL'S  engraving  in  placing  the  "cape"  on  the  n.  f.  side  of  the 
Sinus  magnus;  edge  of  cape,  17"  n.  of  [669],  whereas  it  is  south  as  much  as  this. 
[HERSCHEL'S  drawing  certainly  differs  in  this  respect  from  all  drawings  since  1865.] 

1864,  March  28.  [Memoir,  p.  1864.]  [The  nebula  was  observed  in  the  twilight 
for  the  purpose  of  noting  the  order  of  brightness  of  the  various  parts.] 

7h  -i  7m,  sid.  time.     [A]  and  surrounding  parts  visible. 

7h  igm.  Outline  of  Huyghenian  region  is  readily  distinguished,  especially  near 
685.  [D]  is  a  little  but  not  much  fainter  than  [A]. 

7h  25™.  The  [occiput]  is  evidently  less  clearly  defined  than  the  [frons~\.  Sinus 
Lamontii  is  not  darker  than  the  dark  channel  between  [I  and  (F  and  G)].  I  was  confi- 
dent of  tracing  the  continuation  of  [occiput]  across  the  Sinits  Lamontii. 

1864,  April  7.  R.  Picardiana  immediately  north  of  trapezium  is  far  brighter  than 
the  Messierian  branch.  [This  is  different  from  LIAPONOFF,  p.  79.] 

1864,  April  9.  The  [north]  terminus  of  D  is  quite  sharp. 

1 864,  April  14.  The  limit  of  the  -Huyghenian  region  falls  short  of  the  R.  A.  of  [708]. 

1864,  April  15.  46  of  north  point  of  D  is  measured  6g".S.  [Following  edge  of 
[D]  J6  —  25".6  measured.  646  is  far  within  the  nebulosity.] 

COMETIC  TAILS  TO  685,   708,   741. 

[These  were  always  seen  under"  good  circumstances,  and,  I  believe,  with  various 
eye-pieces.  I  give  below  a  few  of  the  dates  (from  Annals  Harv.  Coll  Obs.,  vol.  v,  pp. 
155  et  seq.)  on  which  mention  is  made  of  them.] 

1859,  March  10. 

1 86 1,  February  6.     A  tail  to  570  noted. 

1864,  January  26.     A  tail  to  746  noted. 

1864,  February  3. 

MEMORANDA    FROM    A    SKETCH    OF    GEO.    P.    BOND'S,   DATED    APRIL    1 6,     1864. 

Sinus  magnus. 

A  rough  sketch  is  given  showing  r'  and  r"  [of  index-chart],  etc.,  and  various 
notes  are  made,  as  follows : 

From  640  a  line  is  drawn  n.  /  in  j>L45°,  and  BOND  says  ««.  p.  this  line  the 
Sinus  magnus  is  filled  with  diffuse  light,  and  edges  are  ill  defined." 


84  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

"Limit  of  dim  outline  of  bay  on  south  shore  of  Sinus  magnus  [i.  e.,  south  shore  of 
T~\  reaches  to  declination  of  #'." 

"General  direction  of  irregular  [southern]  outline  of  Sinus  magnus  (brighter  light) 
*  *  *  *  is  inclined  25°  to  parallel  [^r=ii5°].  This  cuts  off  part  of  decided 
promontory  meeting  bridge"  {(Pans  Schroeteri;)  this  refers  to  R.] 

"Limit  of  dim  outline  of  bay  [r']  in  south  shore  of  Sinus  magnus  reaches  to  10" 
or  12"  south  of  9',  terminates  with  second  bridge"  [by  "second  bridge"  is  meant  the 
preceding  edge  of  4;  and  not  "  SCHROETER'S  second  bridge"].  "  South  limits  estimated 
with  care." 

"Outlines  of  two  bays  dimly  suggested  on  the  north  shore  of  Sinus  magnus  [these 
are  T  and  r'\  Evidently  the  bay  [r'~]  has  to  do  with  the  impression  by  the  channel 
coming  southward,  or  rather  s.  f.  from  [652,  657,  663,  lacus  Lassellii],  and  the  separa- 
tion into  two  bays  is  as  evidently  suggested  by  the  bridge  lpons  Schroeteri.'" 

North  "  outline  of  bay  [r]  dimly  suggested."  ******"  The  above 
sketch  appears  to  me  to  suggest  a  more  correct  outline  of  Sinus  magnus  than  the  lines 
of  my  engraving,  although  in  that  it  is  the  effect  mainly  which  should  be  corrected. 
The  suggestion  of  two  ovals  forming  Sinus  magnus  is  stronger  when  vision  is  bad,  and 
no  doubt  HERSCHEL'S  drawing  [1837]  aims  at  representing  this." 

Lacus  Secchii. 

"  This  prominent  dark  spot  is  the  sudden  limit  of  light  of  Huyghenian  region,  and 
from  thence  to  [D]  the  limit,  though  indented,  is  precisely  on  the  parallel." 

The  parallel  of  i'  north  of  0'  is  nearly  the  limit  of  bright  light  from  the  trapezium 
northward  between  the  R.  A.  of  [647]  and  the  R.  A.  of  the  preceding  edge  of  [lacus 
Secchii]. 

D. 

The  following  side  of  D  "  is  well  defined  and  straight,  forming  nearly  a  straight 
line  in  the  meridian  with  edges  of  masses  south  of  it  [i.  e.,  of  T],  but  the  latter  incline 
more  to  the  south  following"  647  is  immersed  in  D,  and  651  is  on  its  edge. 

Preceding  edges  of  J.  and  B. 

"This  edge  of  nebula  in  meridian  precisely  through  [575]." 

A. 

The  south  point  of  A  runs  up  to  608  pretty  exactly.  621-622-625  appear  to 
be  in  darkness  according  to  this  sketch. 

From  memoranda  of  features  to  be  re  examined.  BOND  remarks  that  the  outline 
of  the  occiput  is  continuous  across  the  mouth  of  Sinus  Lamontii,  although  less  bright. 

In  the  Memoir  "  On  the  Spiral  Structure  of  the  Great  Nebula  of  Orion,"  pre- 
sented by  G.  P.  BOND  to  the  Royal  Astronomical  Society  (Mon.  Not.,  R.  A.  S.,  xxi,  p. 
203),  BOND  speaks  of  the  small  wisps  or  tails  of  light  which  are  shown  most  plainly 
in  his  engraving  in  the  Annals  of  Harvard  College  Observatory,  vol.  v,  near  the  stars 
685,  708,  741  of  his  Catalogue.  That  this  was  really  seen  by  BOND  we  learn  from 
his  accurate  description  of  them ;  he  speaks  of  "  the  large  number  of  instances  in 
which  collections  of  nebulous  matter  are  found  associated  with  stars,  frequently  in  the 


form  of  little  wisps,  shooting  off  in  a  southerly  or  south  preceding  direction."  Other 
things  are  mentioned  which  point  to  a  connection  between  the  stars  and  the  nebula 
as  e.  g.,  "the  predominance  of  small  stars  in  the  nebulous  regions,"  the  "  two  remark- 
able instances  where  there  is  a  deficiency  of  nebulous  matter  in  close  proximity  to 
bright  stars,  which  are  yet  closely  encircled  by  it  These  are  the  bright  groups  of  the 
trapezium,  the  central  comparative  darkness  of  which  has  been  noticed  by  many 
observers,  and  i  Orionis.  Lord  ROSSE'S  figure  of  the  latter  is  decisive  on  this  point 
These  features  seem  to  favor  the  idea  of  a  physical  association  of  the  stars  with  the 
nebula.  The  existence  of  a  spiral  arrangement  of  its  component  parts  falls  in  with 
the  suggestion  of  a  stellar  constitution,  since,  among  the  objects  exhibiting  this  pecu- 
liarity are  included,  not  only  resolvable  nebulae,  but  actual  star-clusters,  such,  for 
instance,  as  the  great  cluster  in  Hercules,  which  has  an  unquestionable  curvilinear 
sweep  in  the  disposition  of  its  exterior  stars. 

In  the  Monthly  Notices  of  the  Royal  Astronomical  Society,  vol.  24,  p.  1 79,  G.  P. 
BOND  replies  with  definiteness  and  with  perfect  justness  to  strictures  which  had  been 
made  upon  his  published  engraving  (same  work  and  vol,  p.  92),  and  institutes  a  com- 
parison between  his  drawing  and  HERSCHEL'S  (1837),  which  had  been  quoted  as  evi- 
dence against  his  own.  I  quote  certain  portions  of  this  as  supplementary  to  what  has 
already  been  given : 

"  The  only  areas  quite  destitute  of  light  which  I  have  found  in  this  part  of  the 
nebula,  are:  ist.  An  irregular  opening  with  its  center  in  the  position  Ja-\- 108",  4d 
+  50"  [i.  e.,  our  r'],  and  2d,  a  narrow  channel  having  its  axis  nearly  in  the  parallel, 
and  a  declination  of  45  —  -f-  72"  at  the  right  ascension  Ja  =  -+-  160""  [our  r"].  *  * 
******  a  -yye  ]iave  a  fi^  weu  defined  point  of  departure  at  the  position 
4 a  —  -J-  145",  48  —  —  20"  [our  point  Q  ;  LIAPONOFF'S  B].  Of  this  there  is  no  trace 
in  HERSCHEL'S  drawing.  LIAPONOFF  gives  z/#  —  14.6". 5,  48  22" .o.  HERSCHEL  makes 
the  breadth  of  the  bright  light  here  still  40"  to  50",  and  continues  the  curve  *  *  * 
180"  beyond  its  actual  limit."  "  We  find,  then,  the  following  instances  of  discrepancy 
between  HERSCHEL'S  delineations  of  the  region  in  question  and  the  actual  appearance 
of  the  nebula : 

"  ist.  The  absence  of  a  definite  limit  to  the  bright  light  of  the  Huyghenian  region 
on  its  eastern  side,  etc. 

"2d.  The  bright  light  on  the  southern  shore  is  carried  10"  to  15"  too  far  north." 
******* 

"4th.  In  its  best  defined  part  the  western  shore  is  placed  12"  too  far  to  the  west. 

"  5tn-   [go]  is  15"  to  20"  too  far  north,  etc. 

"6th.  All  the  features  of  the  northern  shore  [of  Sinus  magnus]  to  the  east  of  pons 
Schroeteri  are  represented  in  positions  30"  or  40"  north  of  their  true  locality  at  the 
same  time  that  the  direction  of  the  principal  lines  is  largely  in  error." 

OBSERVATIONS  OF  LORD  ROSSE  (1867). 

(These  observations  are  extracted  from  Phil.  Trans.,  1868,  p.  57,  et  seq.) 

"  The  observations  upon  this  nebula,  recorded  in  the  journal  of  the  observatory 

at  Parsonstown,  date  from   1849.     From  that  time  till  February,    1858,  there  are 

entries  of  54  observations. 


86 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


In  the  year  1852  Mr.  BINDON  STONEY  made  a  drawing  of  the  Huyghenian  region, 
which  is  a  very  interesting  record.*  Mr.  BLNDON  STONEY  was  a  highly  educated  civil 
engineer,  well  accustomed  to  use  his  pencil. 

His  drawing  was  made  with  great  care,  and  he  was  engaged  upon  it  the  whole 
season.  It  was  compared  by  several  persons  with  the  nebula,  and  was  considered 
exact.  When  we  compare  this  drawing  with  the  nebula  as  it  is  (Figure  33)  at  present, 
there  are  strong  indications  of  change. 


FIG.  33.     ROSSK,  i865-'67. 

Between  February,  1860,  and  February,  1864,  there  are  74  entries  of  observa- 

*  A  photograph  of  this  interesting  drawing  has  been  kindly  sent  me  by  Lord  ROSSK,  to  whom  my  thanks  are  due 
for  this  and  other  similar  kindness. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  g; 

tions.     In  February,  1860,  Mr.  HUNTER,  who  was  then  the  assistant,  being  an  accom- 
plished artist,  commenced  a  new  drawing,  and  was  engaged  upon  it  till  February,  1864 

As  a  groundwork  for  his  drawing,  Mr.  HUNTER  laid  down  all  the  stars  given  in 
"  Observations  de  la  grande  nebuleuse  d'Orion  faites  a  Cazan  et  a  Pulkova,  par  0. 
STRUVE,  St.  P^tersbourg,  1862,"  in  the  positions  given  at  page  1 18  of  that  treatise ;  the 
nebulosity  was  gradually  filled  in  by  eye  as  correctly  as  possible  with  reference  to 
the  stars  given  in  that  memoir,  and  twenty-eight  additional  stars  from  the  9th  to  the 
1 5th  magnitude  were  inserted  by  eye-estimation 

During  the  season  1864-^5  the  nebula  was  often  examined  with  the  view  of 
verifying  the  drawing  made  by  Mr.  HUNTER,  and  in  i865~'66  some  additions  were 
made  to  it. 

During  the  season  1866-67  these  measures  were  completed,  the  additions  of  the 
previous  season  verified,  and  the  drawing  extended.  [The  Figure  33  is  copied  from 
the  drawing  black  on  white  published  in  sections,  and  not  from  the  large  engraving.] 

Very  little  need  be  said  on  this  subject,  as  the  drawing  will  speak  for  itself;  it 
may,  however,  be  well  to  call  attention  to  the  apparent  connection  between  some  of 
the  stars  and  the  nebulosity  near  them. 

In  some  places  the  stars  appear  to  have  either  repelled  or  absorbed  the  nebulosity, 
for  instance  at  the  trapezium,  at  32*  and  35,  and  so  on;  and  in  other  places  the  nebu- 
losity is  denser,  as  if  the  star  had  attracted  it,  for  instance  at  2lt  4,  34,  and  108.  Around 
the  star  108  [734]  the  nebulosity  s6ems  to  have  a  spiral  character,  and  the  same 
appearance,  though  much  less  decided,  may  be  seen  round  4.  Round  the  stars  46^  46U, 
and  991?  the  nebulosity  seems  to  have  been  concentrated,  but  close  to  them  there  appears 
to  be  an  absence  of  nebulosity ;  and  in  the  case  of  99^  the  dark  hole  is  situated  eccen- 
trically with  respect  to  the  principal  star,  its  nearer  companion  being  close  to  the 
opposite  side  of  the  hole;  but  in  the  case  of  the  double  star  46^  46U,  the  hole  is  nearly 
symmetrically  situated,  but  the  nebulosity  is  brightest  at  the  north  preceding  side. 
We  can  hardly,  therefore,  account  for  these  numerous  coincidences,  except  by  sup- 
posing some  at  least  of  the  stars  to  be  situated  nearly  at  the  same  distance  from  us  as 
the  nebula ;  in  fact  immersed  in  the  nebulous  matter. 

[This  point,  as  brought  out  by  Lord  ROSSE,  and  confirmed  as  it  is  by  a  telescopic 
examination,  appears  to  be  a  conclusive  proof  that  we  have,  at  least,  some  of  the  stars 
associated  with  the  nebula.] 

Variability  of  form  and  intensity  of  the  nebulosity. — On  this  subject  it  is  impossible  to 
speak  decidedly.  On  comparing  the  following  six  drawings — 

Sir  J.  HERSCHEL'S  of  about  the  year  1825, 
Sir  J.  HERSCHEL'S  "          "         1837, 

Mr.  BOND'S  1848, 

M.  LIAPONOFF'S  "          "         1850, 

Mr.  LASSELL'S  1854, 

Mr.  HUNTER'S  "          "         1863, 

great  discrepancies  exist  in   almost  every  part,   but   these  are  probably  to  be  at- 
tributed in  a  great  measure  to  the  difference  of  power  in  the  instruments  used  and 

*  Only  those  stars  within  the  limits  Aa  =  ±  300"  and  A,l  =  ±2oo"  have  been  marked  with  BOND'S  number.-E.  S.  H. 


88  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

the  amount  of  labor  expended  on  the  drawings,  as  no  continuous  change  seems  to  be 
shown  by  them.  In  the  case  of  the  spiral  nebula  round  108  [734],  BOND'S,  LASSELL'S, 
and  HUNTER'S  drawings  appear  to  agree  tolerably  well,  allowance  being  made  for  the 
difference  of  size  of  the  instruments,  but  when  we  go  back  to  HERSCHEL'S  drawing  of 
1837,  we  find  a  considerable  discrepancy.  HERSCHEL'S  drawing  of  1825,  however,  as 
far  as  it  goes,  is  in  this  place  more  like  the  latter  drawings.  With  regard  to  the 
following  extremity  of  the  Huyghenian  region,  all  the  former  drawings,  with  the  excep- 
tion of  LIAPONOFF'S,  represent  the  frons  as  curving  round  to  meet  the  Proboscis  major, 
which  latter  also  Curves  round  to  meet  the  former,  whereas  Mr.  HUNTER  represents 
both  these  parts  as  curving  slightly  in  the  opposite  direction.  This  I  am  satisfied  is 
their  present  appearance.  If,  however,  the  night  is  not  good,  they  acquire  very  much 
the  appearance  of  the  other  drawings,  the  light  of  the  brighter  portions  being  scattered, 
to  a  certain  extent,  over  the  intervening  space.  In  the  case  of  the  Huyghenian  region, 
HERSCHEL'S  drawing  (1837)  agrees  much  more  nearly  with  Mr.  HUNTER'S  than  any  of 
the  others,  although  the  interval  (30  years)  is  so  much  longer  than  in  the  case  of  Mr. 
BONDS  and  Mr.  LASSELL'S  drawings  (15  and  9  years,  respectively). 

With  reference  to  the  relative  brightness  of  the  various  parts,  I  find  recorded  by 
Mr.  HUNTER,  February  22,  1861  : 

"  In  bright  moonlight  the  degrees  of  brightness  are — 

"  i.  The  Huyghenian  region. 

"  2.  The  nebulosity  immediately  south  preceding  it. 

"  3.  The  Mairanian  region. 

"  4.  The  subnebulous  region. 

u  5.  The  south  Messierian  branch,  and  the  nebulosity  immediately  north  of  the 
Huyghenian  region" 

And  again :  "  The  observation  of  February  22,  1861,  gives  very  different  degrees 
of  brightness  for  the  various  regions  from  what  they  had  this  season  (i863~'64). 

"  i.  The  Huyghenian  region. 

"  2.  The  nebulosity  immediately  south  preceding  it. 

"  3.  The  nebulosity  immediately  north  of  it. 

"  4.  Subnebulous  region. 

"  5    The  south  Messierian  branch  and  the  Mainmian  region  nearly  equal." 

Mr.  HUNTER  on  two  occasions  estimated,  as  nearly  as  he  could,  the  relative  bright- 
ness of  the  various  masses  of  nebulosity  of  the  Huyghenion  region.  The  following  are 
his  estimations.  (See  diagram.)* 


FEBRUARY  13,  1864. 

a  [A],  r  [L],  v  [Q],  y  \F] 

nearly  equal;  brighest  of  these  is  perhaps  6. 

JT[CJ. 

co  [in  A],  e  [H],  <5  [G],  ft  [I]  ; 

ft  is  the  faintest  of  these  four. 

a  [E],  t  [in  Q],  ^  [2],  A  [in  Q]. 

<?  [in  M],  <p  [between  J  and  A]  faintest. 


MARCH  i,  1864. 
tf  [A],  brightest. 

y  [Q],  r  [L]. 

y  [F],  e  [H],  d  [G]. 
0  [N],  11  [E]. 
7t  [C],  very  faint. 


*  To  these  estimates  we  may  attach  much  importance,  as  Mr.  HUNTKK  had  the  advantage  of  a  considerable  amount 
of  training  as  an  artist. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  89 

"There  are  several  places  where  we  have  reason  to  suspect  that  a  change  of  form 
may  have  taken  place  in  the  nebulosity  since  our  observations  commenced : 

i  st.  In  Mr.  BINDON  STONEY'S  drawing,  of  which  an  outline  is  given  at  the  upper 
right-hand  corner  of  the  skeleton  map,  a  dark  line  exists  running  from  88  [671]  in  a 
direction  parallel  to  the  from,  whereas  at  present  the  only  break  in  the  nebulosity 
at  all  in  the  same  direction  runs  from  88  [671]  in  a  south  following  direction.  [It  is 
thus  at  present] 

2d.  The  projection  of  the  nebulosity  below  88  [671]  into  the  Sinus  magnus  does 
not  exist  in  Mr  STONEY'S  drawing,  [ju  of  ROSSE  —  R  !] 

3d.  The  following  outline  of  the  nebulosity  immediately  below  75  [647]  is  con- 
cave towards  the  following  side  in  Mr.  STONEY'S  drawing,  but  convex  in  Mr.  HUNTER'S 
[also  in  Washington  Observations].  In  all  these  points  I  believe  that  Mr.  HUNTEB 
gives  as  nearly  as  possible  the  present  appearance. 

4th.  Mr.  HUNTER  represents  the  outline  of  the  nebulosity  surrounding  the  dark 
region  or  lake  round  the  stars  32  [449],  35  [479],  as  very  marked.  I  often  examined 
this  part  during  the  seasons  1864-65  and  1865-^6,  but  never  saw  it  quite  as  distinctly 
as  it  is  represented  on  the  following  side,  nor  did  I  see  the  elbow  just  following  35  ; 
the  nebulosity  appeared  to  be  more  of  the  shape  represented  by  the  coarsely  dotted 
line  in  the  skeleton  map. 

5th.  I  was  never  able  to  see  more  than  two  of  the  three  rays  below  this  lake,  and 
except  on  two  or  three  occasions  I  could  only  make  out  one.  Mr.  HUNTER  has  since 
told  me  that  in  the  last  season  during  which  he  was  working,  these  rays  were  much 
fainter  than  they  had  been  previously,  and  that  they  are  represented  too  bright  for  their 
appearance  during  the  season  i863~'64. 

In  connection  with  this  subject,  it  may  not  be  uninteresting  to  compare  the  obser- 
vations of  former  observers  with  each  other  and  with  our  own. 

Sir  J.  HERSCHEL  in  his  paper  of  1825  discusses  the  differences  between  his  own 
drawings  and  those  of  HUYGHENS,  PICARD,  MESSIER,  and  LE  GENTIL,  and  thinks  that 
the  first  three,  when  compared  with  his,  tend  to  show  a  gradual  diminution  or  conden- 
sation of  the  nebulosity ;  but  LE  GENTIL'S,  which  was  older  than  MESSIER'S,  represents 
it  just  as  he  himself  saw  it. 

We  next  come  to  Sir  J.  HERSCHEL'S  paper  of  1837,  in  which  he  says  that  although 
to  any  one  who  has  not  viewed  this  object  through  powerful  telescopes  the  differences 
between  the  various  drawings,  including  his  own  of  1824  and  1837,  may  seem  great, 
and  tend  to  convey  an  impression  of  great  and  rapid  changes  undergone  by  the  nebula 
itself,  yet,  after  carefully  comparing  his  own  two  drawings,  he  comes  to  the  conclusion 
that  the  differences  are  not  greater  than  he  is  disposed  to  attribute  to  his  own  inexpe- 
rience in  such  delineations  in  1824,  to  the  greater  care  bestowed  on  the  later  drawing, 
and  especially  to  the  advantage  of  better  local  situation  and  superior  defining  power, 
etc.,  of  the  telescope  at  the  latter  date  (Cape  Observations,  page  31).  There  are  three 
points,  however,  to  which  he  directs  attention,  but  in  the  case  of  two  only  of  them  is 
he  inclined  to  conclude  that  there  is  any  evidence  of  change;  these  points  are— 

i.  The  form  and  position  of  the  nebula  oblongata  between  127  and  129. 
12 


9O  MONOGRAPH   OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

2.  The  position  of  the  nebulous  spur  between  1 1 1  and  122. 

3.  The  form  of  the  nebula  round  108. 

In  1824,  Sir  J.  HERSCHEL  saw  the  nebula  oblongata  as  a  " tolerably  regular  oval," 
nearly  in  a  line  between  the  stars  120  [781]  and  136  [848],  whereas  in  his  drawing 
of  1837  it  is  irregular  in  outline,  and  decidedly  above  the  line  through  120  and  136 

With  respect  to  the  form  of  the  nebula  oblongata,  the  brighter  part  forms  a 
"  tolerably  regular  oval,"  but  when  the  fainter  parts  are  included,  it  seems  to  be  more 
of  the  form  given  in  HERSCHEL'S  drawing  of  1837.  It  is,  therefore,  quite  possible, 
even  probable,  that  HERSCHEL  would  have  seen  it  oval  in  1825,  but  long  and  slightly 
curved  upwards,  with  the  superior  means  at  his  disposal,  in  1837,  without  any  change 
of  form  having  taken  place  in  the  interval;  but  as  regards  its  position,  it  appears  to  be 
now  entirely  above  the  line  120 — 136. 

With  regard  to  the  nebulous  spur  between  111  [746]  and  122  [783],  diagrams 
which  he  made  in  1832  and  1834  represent  it  as  "running  directly  from  135  to  i  n 
and  forming  a  complete  hook  no  way  disjoined  from  the  proboscis."  In  1837  he  saw 
it  "neither  joined  to  the  proboscis  nor  directed  towards  135,  but  rather  towards  a 
point  one-third  the  distance  from  135  to  126,"  near  the  position  of  131.  HERSCHEL'S 
second  drawing  appears  to  agree  very  fairly  with  the  accompanying  one  in  this 
respect;  perhaps  the  superior  definition. of  HERSCHEL'S  instrument  in  1837,  a  better 
atmosphere,  and  the  greater*  meridian  altitude  of  the  object  enabled  HERSCHEL  to 
perceive  the  interval  between  this  spur  and  the  proboscis  which  had  escaped  his  notice 
in  1832  and  1834. 

With  regard  to  the  nebula  round  108,  the  amount  of  detail  in  HERSCHEL'S 
drawing  of  1837  is  so  much  greater  than  in  that  of  1824,  and  the  detail  in  the  accom- 
panying drawing  is  so  much  greater  than  in  HERSCHEL'S  of  1837,  that  it  seems  hardly 
possible  to  arrive  at  any  conclusion  by  comparing  them. 

The  engraving  is  upon  the  whole  very  accurate;  a  little  more  softening  off  in  the 
faint  outlying  parts  would  have  been  desirable,  but  Mr.  BASIRE  did  not  think  that  it 
would  be  practicable,  consistent  with  the  reasonable  durability  of  the  plate;  the  forms, 
however,  are  correct.  The  sharpness  of  outline  and  the  hard  and  marked  character  of 
the  principal  features  are  the  result  of  the  great  light  of  the  instrument ;  with  a  dimin- 
ishing aperture,  these  characteristics  gradually  fade  away.  The  engraving  faithfully 
represents  the  object  as  it  may  be  seen  on  any  clear  night,  and  the  details  are  so  well 
marked  that  no  material  change  can  take  place  hereafter  which  will  not  at  once  be 
recognized  with  an  instrument  of  similar  power.  The  interior  of  the  trapezium  has 
not  been  examined  recently  with  the  view  to  the  question  whether  it  is  absolutely 
dark.  With  the  6-foot  instrument  the  eye  is  so  dazzled  by  the  light  of  the  four  stars 
that  it  is  difficult  to  form  an  accurate  opinion;  and  any  nebulosity  which  may  exist  is 
probably  too  faint  to  affect  the  spectroscope.f  I  am  not  certain  that  any  part  of  the 
nebula  is  absolutely  free  from  nebulosity,  but  the  contrast  is  so  great  between  the 
dark  spaces  alluded  to  by  Sir  JOHN  HERSCHEL  and  the  contiguous  portions  of  the 
nebula,  that  even  in  the  drawing  it  was  scarcely  possible  to  indicate  nebulosity  so 

*  This  last  applies  to  the  diagram  of  1832  only.  t  See  Memoirs  by  HUYGHENS,  LE  SUEUK,  D'ARREST,  and  others. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  91 

slightly  as  not  to  interfere  with  the  proper  gradation  of  light ;  in  fact  it  was  scarcely 
possible  to  represent  the  bright  parts  sufficiently  bright. 

OBSERVATIONS  OF  SECCHI  (1868). 

Following  I  give  a  translation  of  portions  of  SECCHI'S  memoir: 
"Sinus  magnus. — The  bottom  of  this  gulf  is  separated  from  the  region  of  the  trape- 
zium by  two  opposite  promontories  of  moderate  brightness  and  almost  triangular 
form  which  close  it 
by  their  juncture. 
We  have  already 
spoken  of  the  infe- 
rior one  F  [the  «0 
of  LIAPONOFF  D]; 
the  other  opposite 
is  much  less  lumi- 
nous. The  bottom 
of  the  gulf  is  sepa- 
rated from  the  rest 
by  the  bridge  of 
Schroeter,  the  varia- 
bility of  which  is, 
it  seems  to  me,  in- 
dubitable. In  HER- 
SCHEL  this  bridge  is 
indicated  as  a 
simple  promontory, 
and  on  slightly 
cloudy  evenings 
this  has  been  its 
appearance,  but 
when  it  has  been 
clearer,  it  has  al- 
ways been  visible 

as  a  true  bridge  formed  by  light  veils  of  mist  that  traverse  the  gulf.  And  in  a  former 
year  I  find  that,  in  this  connection,  a  correction  was  made  by  hand  to  a  drawing  of 
HERSCHEL'S  with  an  express  reference  of  this  nature  in  the  journal  of  observation.  Prior 
to  that  we  noted,  by  hand,  upon  the  drawing  of  HERSCHEL  a  bright  point,  not  stellar,  in 
the  middle  of  the  bridge,  which  was  not  visible  when  our  drawing  was  finally  made  (g0). 
Afterwards  from  the  bridge  down  to  the  bottom  of  the  gulf  there  was  to  be  seen  in 
1857  a  continuous  nebulosity,  not  so  thin,  a  drawing  of  which  was  made  with  much 
diligence,  precisely  because  it  was  not  to  be  seen  in  the  drawing  of  BOND,*  on  which  it 
was  drawn  by  hand ;  it  terminated,  however,  in  an  arc,  convex  toward  the  bottom  of 
the  gulf  itself.  In  1867,  on  the  other  hand,  the  mist,  if  not  vanished  altogether,  had 
at  least  diminished  very  considerably;  and  the  bridge  seems  curved  inversely  and 

*  All  references  by  SKCCIII  to  "  BOND"  are  to  W.  C.  BOND,  1848. 


FIG.  34.    SECCHI,  1868. 


92  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

almost  uninterruptedly,  tracing  a  rudimental  spiral  arc.  In  the  drawing  of  1857,  I 
find  but  a  very  faint  trace  of  the  other  branches  that  may  be  seen  as  promontories  in 
the  gulf  following  the  principal  bridge,  and  that  now  have  become  brighter.  But  a 
little  mist  in  the  atmosphere  causes  all  these  minute  details  to  disappear.  The  bottom 
of  the  gulf  being  free  from  stars,  its  luminous  appearance  cannot  be  attributed  to  the 
influence  of  the  atmospheric  air.  LTAPONOFF  has  drawn  a  more  brilliant  luminous 
mass  in  g0,  near  the  middle  of  the  bridge,  which  would  seen  to  confirm  the  one  seen 
by  us. 

On  the  28th  February,  1865,  we  observed  the  nebula  together  with  STRIJVE,  and 
we  were  surprised  to  find  the  mouth  of  the  Sinus  magnus  shut  in  by  nebulosity  in  such 
a  way  that  the  anterior  part  of  it  appeared  like  a  great  closed  (oval)  ellipse,  whence 
uniting  to  this  the  part  following  the  bridge,  all  the  gulf  had  the  appearance  of  the  figure 
8  or  of  a  Lemniscata.  This  form  had  already  been  seen  by  us,  likewise,  in  1858,  on  the 
1 5th  January,  and  noted  as  extraordinary,  and  as  discrepant  from  another  drawing  of 
this  part  made  previously.  I  read  in  the  journal:  The  nebula  is  well  drawn;  it  only 
lacks  a  little  nebulosity  in  the  opening.  The  internal  area  (as  far  as  the  bridge)  is  black,  and 
almost  exactly  round.  The  comma  (nebula  of  MAIRAN)  is  composed  of  two  pieces.  Here 
there  is  no  mention  made  of  the  second  rudimentary  bridge,  which  is  found  not  only 
in  the  drawing  of  Father  FERRARI,  but  likewise  in  my  sketch  in  1867.  [SCHROETER'S 
second  bridge  in  T  is  not  here  referred  to. — E.  S.  H.]  This  nebulosity  in  the  mouth 
(opening)  is  not  to  be  confused  with  the  nubecula  minor,  which  stands  in  front  of  it, 
but  at  some  distance,  of  which  a  trace  is  found  in  HERSCHEL,  but  which,  according 
to  him  proceeds  to  unite  with  the  peninsula  of  the  region  of  PICARD,  though  I  find  no 
correspondence  with  these  convolutions  in  our  work. 

The  great  gulf,  which  gives  to  the  nebula  the  form  of  a  monstrous  head  with 
wide  open  jaws,  is  reproduced  in  all  (the  old)  drawings.  By  reason  of  the  weakness 
of  the  instruments  used,  those  drawings  do  not  merit  discussion,  but  it  is  curious  to 
note  in  them  the  difference  in  the  aperture  of  the  gulf  and  the  length  of  the  jaws. 
Probably  by  imitating  the  old  instruments  in  various  conditions  of  the  air  one  might 
succeed  in  obtaining  the  same  appearances,  and  certainly  nothing  could  justify  the 
enormous  variations  that  those  configurations  suppose,  judging  from  what  we  saw  here 
forty  years  ago. 

Regio  Picardina — Its  extremity,  K,  forms  a  peninsula  which  has  the  form  of  a 
point  of  an  arrow  directed  obliquely  downward,  and  by  moonlight  is  seen  separated 
from  the  rest  by  the  lacus  Lassellii,  which  communicates  with  the  Sinus  magnus  by 
means  of  a  dark  canal.  But  on  a  dark  night  no  true  interruption  exists.  This  penin- 
sula has  three  well-defined  points  on  the  side  of  the  gulf,  the  middle  one  of  which 
is  the  brightest.  Only  on  one  occasion  the  lacus  Lassellii  failed  to  be  marked  upon 
one  of  our  drawings,  but  the  fact  that  the  necessity  for  correction  was  felt  imme- 
diately after,  proves  that  the  omission  was  accidental.  There  is  a  trace  in  HERSCHEL  of 
the  lacus,  but  the  ingress  to  it  is  barred  by  a  nebulous  zone  without,  which  is  certainly 
not  seen  now. 

Lacus  Secchii  was  discovered  by  0.  STRUVE,  and  is  perfectly  visible  but  always 
difficult  to  recognize  in  the  splendor  of  this  region.  Sometimes  I  have  seen  it  very 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

clearly,  again  quite  indistinctly,  and  it  is  difficult  to  indicate  the  reason  of  this.  The 
bottom  is  nebulous  and  the  margins  poorly  denned,  and  its  contour  is  rudely  cruci- 
form. In  2  there  is  a  similar  spot,  analogous,  previously  seen  by  STRUVE,  which  might 
be  called  lacus  Struvii,  but  it  is  less  clearly  denned  than  the  preceding  one.  Near  its 
following  margin  it  has  the  little  star  70  [635]  of  HERSCHEL,  and  another,  similar 
stands  near  the  other  small  star,  though  eccentrically  to  it. 

Going  towards  the  left,  in  the  direct  line  of  the  two  lakes  S  and  2,  one  comes  to 
the  palm  Bondii,  which  is  terminated  by  two  circular  arches  The  most  southern  of 
these  arches  is  far  more  brilliant  than  the  northern.  These  two  hemicycles  are  sepa- 
rated by  a  narrow  canal  seen  by  STRUVE,  from  which  the  palus  communicates  with  the 
Sinus  Lamontii,  and  by  it  with  the  region  of  LE  GENTIL. 

Eegio  Gentiliana. — LE  GENTIL  was  the  first  who  saw  above  the  region  of  HUYGHENS 
a  black  oblique  canal,  which  was  afterwards  better  traced  by  LAMONT.  The  figure  of 
LE  GENTIL  presents  it  drawn  with  hard  [border]  lines,  altogether  unlikely  in  so  neb- 
ulous an  object.  The  Sinus  Gentilii,  which  forms  a  bay  beyond  the  canal  of  LAMONT, 
ordinarily  is  entirely  free  from  nebulosity.  Through  this  vestibule  one  enters  the 
canal  of  LAMONT,  which  is  distinct  in  two  successive  almost  semi-circular  spaces. 

There  is  a  trace  in  HERSCHEL  of  the  vestibule  and  also  of  the  first  semi-circle, 
but  none  of  the  second.  On  certain  evenings  this  canal  is  so  black  that  it  is  surprising 
it  should  not  have  been  seen  by  every  one,  and  that  explains  the  hard  and  strange 
contour  of  LE  GENTIL.  On  the  evening  of  the  23d  of  January,  1859, 1  find  a  drawing 
made  by  Father  CAPPELLIETTI,  and  retouched  by  me,  in  which  the  Sinus  Lamontii  is  quite 
free  of  nebulosity.  In  the  same  drawing  the  palus  Bondii  is  fully  illuminated,  and 
the  Sinus  magnus,  on  the  contrary,  has  the  bottom  shrouded  from  the  bridge  ofSchroeter, 
upward.  In  another  quite  large  drawing,  done  by  myself  in  white  on  a  dark  blue 
surface,  I  find  that  the  canal  of  LAMONT  is  quite  well  defined,  but  it  lacks  the  point  in 
the  middle  so  that  the  two  hemicycles  form  but  one  of  larger  size,  with  a  brilliant  mass 
in  their  midst,  which  is  what  now  forms  the  midmost  promontory.  Whether  these  divers 
forms  be  one  and  all  attributable  to  accidental  causes,  or  to  real  mutations,  I  am  not  able 
to  decide,  but  I  find  that  STRUVE  does  not  believe  this  to  be  the  condition  of  this  canal. 
In  BOND  there  are  traces  of  it,  but  not  well  defined.  The  gulf  and  the  canal  are 
quite  easily  recognizable  in  the  drawing  of  DE  Vico,  though,  owing  to  the  weakness 
of  his  instrument  (6  inches),  it  lacks  many  details.  All  this  part  has  an  illumination 
inferior  to  the  region  of  HUYGHENS,  and  in  feeble  moonlight  it  seems  almost  as  limited 
as  in  DE  Vico,  while  by  the  full  moon  it  approaches  the  figure  of  LASSELL.  This 
proves  the  enormous  influence  of  the  strength  of  the  instruments  upon  these  contours. 
However,  the  canal  of  LAMONT  is  indirectly  indicated  in  all  those  figures  which  cause 
the  region  of  HUYGHENS  to  terminate  brusquely  in  a  triangle.  I  said  that  there  is  not 
a  trace  of  the  second  semi-circle  in  the  canal  of  LAMONT  in  HERSCHEL  ;  but  this  gen- 
tleman kindly  showed  me  an  unpublished  drawing,  in  which  there  is  some  indication 
of  it,  and  if  he  did  not  publish  it,  it  was,  perhaps,  because  he  did  not  see  it  confirmed. 
But  to  have  drawn  it  is  a  proof  of  having  seen  it,  and  in  short,  all  goes  here  to  prove 
that  we  have  to  do  with  a  region  which  is  either  variable  or  that  merits  further  ulte- 
rior study. 


94 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


Regio  Derhamiana. — This  is  situated  above  the  canal  of  LAMONT.  It  has  a  tolerably 
brilliant  light,  but  is  inferior  to  the  region  of  HUYGHKNS  It  exhibits  quite  a  curious 
network  of  masses  to  us,  among  which  is  a  semi-circle  or  arc  T.  U.  in  the  polygon, 
page  14,  which  appears  to  be  the  hemicyclmm  Liapanouvii  of  STRUVE  [?].  I  find  the 
two  bright  masses  without  this  arc  quite  well  indicated  in  a  drawing  of  1857.  This 
reo-ion  is  somewhat  difficult  to  examine  with  our  instrument,  and  the  gradation  of  light 
is  not  so  sensible  as  in  the  other  portions.  The  disagreements  with  the  other  draw- 
ings are  notable.  .Probably  because  in  the  stronger  instruments  the  augmented  light 
brin'gs  the  less  clearly-defined  lower  regions  into  prominence,  and  thus  causes  the 
more  brilliant  portions  to  disappear,  in  such  a  way  that  the  aperture  and  strength  of 
the  instruments  would  perform,  up  to  a  certain  point,  a  contrary  office  to  that  of  the 
moon,  and  further  studies  are  therefore  rendered  necessary.  Perhaps  this  circumstance 
explains  certain  hard  contours  given  by  several  observers. 

Regio  Messieriana. — Returning  now  to  the  side  toward  the  right,  and  most  distant 
from  the  region  of  HUYGHENS,  we  have  to  consider  here  the  great  proboscis  which  does 
not  enter  entirely  into  the  square  of  our  drawing.  On  this  proboscis  two  well-defined 
promontories  are  projected.  The  principal  of  these,  called  the  promontorium  Herschelii 
up  to  the  year  1857,  has  its  vertex  below  the  neighboring  small  star,  as  in  the 
actual  drawing.  It  is  placed  differently  by  BOND,  who  puts  the  point  against  the 
little  star,  and  by  HERSCHEL,  who  makes  the  point  more  obtuse.  In  the  cavity  of 
the  arc  is  the  second  minor  promontory,  which  is  not  found  at  all  in  BOND,  and  is 
differently  outlined  in  HERSCHEL.  The  form  of  this  second  promontory  traced  in  the 
drawing  of  1857  approaches  more  nearly  to  that  of  HERSCHEL. 

The  proboscis  major  is  not  joined  immediately  to  the  region  of  HUYGHENS,  but  is 
detached  from  the  root  of  it,  and  the  direct  prolongation  of  the  above-named  region  is 
more  properly  in  the  proboscis  minor.  The  three  parallel  stars,  e,  f,  g  (685,  708,  741) 
of  STRUVE  are  all  outside  of  the  denser  nebula  above  the  root  of  the  proboscis,  but  are 
shrouded  in  a  thin  mist.  Thence  the  uncertainty  of  their  positions  with  instruments  of 
small  size  and  little  .strength,  which  are  liable  to  show  them  wrapped  in  the  principal 
nebula,  which  explains  the  figures  of  HUYGHENF  and  of  PICARD.  That  of  LK  GEN- 
TIL,  which  places  them  entirely  outside  (whilst  in  another  drawing  he  places  them 
within),  merits  little  faith  for  other  qualities.  M.  D'ARRFST  has  found  a  figure  of 
LEFEBVRE  which  places  them  outside.  To  mv  mind  all  these  differences  lead  to  the 
conclusion  that  they  proceed  from  differences  in  strength  of  instrument  or  in  atmos- 
pheric clearness.  Only  a  little  power  being  employed  the  subdued  luminous  mist  that 
enwraps  the  stars  is  not  visible,  and  they  appear  to  be  without.  With  good  vision, 
and  an  instrument  which  brings  the  light  of  the  nebula  in  this  part  into  prominence, 
one  would  judge  them  to  be  within. 

In  this  great  gulf,  among  the  probosces  and  the  region  of  HUYGHENS,  there  are 
really  three  gradations  of  light,  and  a  little  above  the  three  stars  before  mentioned  the 
feeblest  light  begins  and  extends  to  a  very  great  distance,  with  an  almost  uniform 
density,  but  it  has  not  been  studied  sufficiently  by  us,  on  account  of  its  dimness. 

Nebula  Mairanii. — The  little  star  marked  jn  by  HERSCHEL,  situated  in  N>£,  2}^, 
which  MAIRAN  saw  surrounded  by  mist,  has  certainly  a  pretty  decided  outline  in  the 


MONOGRAPH  OF  THE  CENTRAL  PARiS  OF  THE  NEBULA  OF  ORION.  95 

form  of  an  inverted  comma.  On  March  31,  1856,  I  find  noted  that  this  mist  has  an 
undoubted  spiral  aspect,  notwithstanding  the  tail  is  separated  by  a  thinner  veil,  which 
makes  it  at  times  appear  detached  and  double.  The  convolutions  indicated  by  HtR- 
SCHEL  and  by  BOND  are  different  from  ours,  and  do  not  agree  among  themselves. 

Between  this  nebula  and  the  region  of  PICARD  of  the  principal  nebula  is  the 
nubecola  oUongata  of  HERSCHEL,  which  almost  touches  the  other  small  star,  itself  also 
surrounded  by  mist  The  most  singular  thing  is  that  the  space  which  separates  this 
nebula  has  been  found  perfectly  black  by  us,  so  that  it  was  absolutely  necessary,  on 
those  evenings  when  we  noted  the  fundamental  forms,  to  cancel  utterly  from  the  map 
every  trace  of  thin  mist  which  had  previously  been  drawn  upon  it.  I  was  surprised 
by  so  much  darkness,  and  took  note  of  it.  This  canal,  so  straight  and  black,  was 
certainly  the  one  which  at  the  end  of  the  last  century  determined  the  giving  to  the 
nebula  a  figure  of  a  capital,  elongated  omega  (see  fig.  of  1 774),  and  thus  it  really  does 
present  itself  in  a  small  instrument  which  does  not  separate  the  details. 

However,  this  is  so  much  the  more  singular  since  O.  STROVE  says  positively  that 
he  saw  some  nebulosity  between  the  region  of  PICARD  and  the  nebula  oblongata,  and  to 
us  as  well  it  would  have  appeared  nebulous  had  we  not  traced  that  mist  which  subse- 
quently we  were  obliged  to  cancel.  Here,  then,  is  one  of  those  points  to  be  re-exam- 
ined in  the  future." 

SECCHI  sums  up  his  results  as  follows: 

"ist.  From  the  comparison  of  our  observations  with  those  of  preceding  astron- 
omers, it  appears  that  the  nebula  is  sufficiently  known  in  its  general  structure.  The 
coincidence  of  the  principal  points  is  now  assured,  and  their  relative  variability  remains 
only  to  be  fixed  by  more  exact  measurements.  The  labors  of  LIAPONOFF,  STRUVE, 
BOND,  and  HERSCHEL,  confronted  with  ours,  put  the  latter  beyond  controversy  as  to 
the  points  of  greatest  brightness  and  of  the  first  order. 

2d.  The  differences  which  are  met  with  occur  principally  in  the  parts  of  the 
second  order,  where  the  weakness  of  the  light,  the  power  of  the  instruments,  the  sensi- 
tiveness of  the  observer's  eye,  and  the  state  of  the  heavens  exercise  an  immense  influ- 
ence. The  nebula  being  green,  all  eyes  have  not  the  same  sensitiveness  for  this  color, 
and  considerable  diversities  in  the  drawings  must  follow.  Taking  all  these  circum- 
stances into  account,  the  divergencies  will  be  seen  to  sensibly  diminish 

3d.  It  is  not  yet  proved,  however,  that  in  these  parts  all  the  differences  are 
effected  by  the  extrinsic  causes  mentioned  above,  and  that  some  real  variation  may 
not  take  place.  Worthy  of  principal  consideration  are  the  Sinus  Lanxmtii  and  the 
pons  Schroeteri,  where  it  is  difficult  to  attribute  everything  to  accidental  and  extrinsic 

variations. 

4th.  The  resolution  into  little  stars  of  the  bright  masses  does  not  at  all  prove  that 
they  are  agglomerations  of  true  stars  The  spectral  diversity  is  fundamental,  and  that 
assures  us  that  the  physical  constitution  of  nebulous  bodies  is  gaseous.  The  form 
with  distinct  points  may  occur  in  a  mass  even  of  this  nature  and  give  rise  to  a  similar 
appearance,  but  from  this  alone  a  true  stellar  constitution  cannot  be  concluded.  A  sim- 
ilar concentration  occurring  in  any  less  dense  part  may  have  given  origin  to  some  of 


96  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

the  bright  points  observed  as  stars  by  more  than  one.  The  very  absence  of  con- 
tinuance in  these  appearances  proves  the  justice  of  this  explanation. 

5th.  The  mass  being1  recognized  as  gaseous,  it  is  impossible  that  it  should  be  in 
constant  equilibrium,  hence  nothing  is  more  easy  to  explain  by  real  movements,  not 
only  the  aforesaid  agglomerations,  but  also  a  large  part  of  the  variations  presented  in 
its  aspect. 

6th  The  confusion  which  is  felt  on  first  observing  the  nebula  is  only  an  ordinary 
case  of  what  happens  at  the  first  sight  of  an  irregular  object  before  one  has  become 
accustomed  to  it  and  has  made  of  it  a  certain  order  of  regular  figures.  This  happens 
even  when  looking  at  the  starry  heavens  before  knowing  the  constellations ;  and  in 
the  (stellar  groups)  clusters,  before  studying  them,  all  appears  confusion,  and  not 
until  after  a  certain  time  does  order  and  regularity  seem  to  enter.  Then,  however, 
the  number  of  the  objects  seems  to  decrease,  but  this  diminution  is  only  apparent. 
Artists  know  in  practice  this  effect,  and  it  is  not  to  be  feared  as  a  defect  that  a  drawing 
made  after  much  study  should  seem  to  decrease  in  parts  because  a  certain  order  is 
gained. 

7th.  The  nebula  of  Orion  is  not  the  only  mass  of  cosmic  matter,  gaseous  and  irreg- 
ular, which  occupies  space  ;  it  is  only  one  of  the  more  dense  agglomerations  of  cosmic 
matter  which  extend  in  some  parts  of  the  heavens,  specially  in  Sagittarius  and  in  Orion. 
If  the  general  clearness  of  the  heaven  prevents  the  discernment  of  the  presence  of  this 
matter,  it  can,  however,  be  shown  by  various  devices,  and  by  the  help  of  certain  spaces 
in  which  its  presence  is  wanting.  The  great  zone  which  extends  over  these  regions 
seems  to  protract  itself  even  to  the  northern  hemisphere  in  which  the  black  space  or 
coal-sack  in  Cygnus  may  easily  be  carried  out  between  the  Milky  Way  and  a  luminous 
zone  in  continuation  of  those  of  Orion,  to  the  pole  in  form  of  a  very  elongated  M- 
Hence,  the  splendor  of  the  heavens  is  due  in  these  regions  not  only  to  the  stars,  but  to 
an  immense  stratum  of  nebula  in  which  our  solar  and  stellar  system  is  immersed. 

8th.  This  matter  interposing  itself  between  us  and  the  stars  may  give  origin  to 
the  aureoles  which  surround  them  in  some  portions  and  influence  their  spectrum. 
Probably  the  green  color  of  the  stars  in  the  neighborhood  of  Orion,  and  the  extreme 
fineness  of  their  black  lines,  depends  upon  this  stratum  whose  action  tends  to  paralyze 
the  effect  of  the  absorption  of  the  atmosphere  proper  of  the  stars  Sirius,  which  does 
not  share  this  influence,  would  be  outside  of  this  mass. 

9th.  Yet  if  these  masses  are  destined  some  day  to  form  stars,  there  is  very  little  hope 
that  we  can  ever  calculate  the  successive  phenomena  to  be  developed  in  such  concen- 
tration. If  the  relations  of  time  and  space  are  in  equal  proportions,  the  movements 
here  must  be  of  the  slowest,  and  the  variation  imperceptible  beyond  our  imagination. 
The  supposed  enormous  mutations  in  other  objects  is  less  believed  in  proportion  to 
the  more  powerful  means  and  more  diligent  care  used  in  computing  the  forms,  and 
what  remains  of  uncertainty  is  more  due  to  the  imperfection  of  the  study  given  than 
to  the  well-proved  reality  of  changes." 

After  this  was  published,  SECCHI  received  a  proof  of  the  engraving  of  Gr  P.  BOND 
(frontispiece),  and  he  compared  this  critically  with  his  own  work. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  97 

NOTES  ON  BOND'S  ENGRAVING. 

"  First  of  all  BOND  recurs,  in  the  region  Huygheniana,  to  the  system  of  superposed 
cumuli  as  in  HERSCHEL,  the  different  series  of  which  form  a  pyramidal  and  almost 
imbricated  figure.  The  three  orders  of  cumuli  essentially  agree  with  our  series,  except 
in  the  special  details  obtained  by  us  during  moonlight ;  and  as  he  retains  the  too 
small  scale  used  in  the  other  drawing,  what  we  said  of  the  cumuli  of  HERSCHEL  is 
applicable  here. 

Their  flat  forms  and  the  narrow  canals  which  divide  them  are  due  certainly  to  the 
great  power  of  his  instrument  ( 1 6  inches),  which  by  increasing  the  light  very  much  in  the 
fainter  parts,  causes  the  half  shades  of  the  globular  forms,  which  come  out  better  in  the 
moonlight,  to  disappear.  It  is  singular  that  BOND,  who  had  often  observed  in  the 
twilight,  should  not  have  noted  this  difference  of  intensity.  But  he,  also,  was  a  victim 
to  the  common  prejudice  of  observing  nebulae  in  complete  darkness  in  order  to  see  it 
better. 

The  figure  being  positive,  that  is,  the  ground  of  the  plate  being  black  and  exquis- 
itely cut,  is  eminently  suitable  for  an  exact  comparison  with  the  sky.  We  have  given 
the  reasons  why  we  could  not  use  this  system. 

The  Sinus  magnus  is  barred  towards  the  bottom  by  the  pons  ScJiroeteri  that  has  a 
luminous  mass  in  the  middle,  exactly  as  we  said  in  our  Memoria,  at  page  20,  it  was  seen 
by  us  in  1857,  but  which  we  have  not  succeeded  in  seeing  again  since.  The  epoch  of 
BOND'S  drawing  being  given  as  1859  to  1863,  would  be  a  strong  confirmation  of  the 
variability  of  this  bridge  and  of  the  precision  of  our  former  observations.  Hence,  it  is 
clear  that  this  portion  of  the  nebula  should  be  watched.  The  bottom  of  the  gulf  from 
the  bridge  up  is  nebulous,  as  we  also  found  formerly.  It  has  a  little  nebulosity  at  the 
mouth,  but  is  not  barred  as  at  'present. 

The  Sinus  Lamontii  is  quite  black  and  has  a  double  curvature  on  the  left  side,  but 
is  wider  at  the  mouth  than  we  found  it  to  be ;  it,  however,  approaches  more  nearly  to 
our  figure  than  other  drawings.  This,  also,  is  a  region  to  be  watched. 

In  the  region  palm  Bondii  long,  continuous,  spiral  filaments  are  found,  which, 
however,  occupy  the  whole  region  Picardiana  and  Derhamiana.  They  have  a  pro- 
nounced spiral  inclination,  and  start  from  side  57  of  our  polygon  from  below  the  prin- 
cipal of  left  base  of  the  large  triangle  Huygheniana  near  the  trapezium.  The  author 
states  that  it  cost  him  much  labor  to  trace  these  spiral  convolutions  in  the  midst  of  the 
labyrinth  of  the  nebulous  mass.  If  we  should  sincerely  express  our  opinion,  however, 
we  believe  that  this  preconceived  idea  of  reducing  the  nebula  of  Orion  to  nebulous 
spirals,  applying  to  it  the  principle  of  Lord  ROSSE,  may  have  slightly  forced  the 
observer's  judgment,  inducing  him  to  give  prominence  to  certain  traits  which,  perhaps, 
have  not  all  the  strength  that  they  show  in  the  drawing.  The  reticulation  in  the 
region  H,  G,  F,  K,  of  our  polygon,  is  certainly  very  confused,  and  a  preconceived 
idea  can  easily  distort  the  fancy;  but  we  do  not  remember  ever  to  have  seen  lines  so 
continuous  and  easy  to  trace  as  those  drawn  by  the  illustrious  deceased,  and  tlu-y 
cannot  be  imagined  from  our  drawing. 

In  BOND'S  drawing  the  large  arc  (H,  7)  of  the  region  Fonrhunm  is  well  traced,  and 
App.  V 13 


g%  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

continues  in  a  large  oval  that  has  in  the  middle  the  brighter  mass  represented  by 
us  in  (D,  3  a  7).  As  our  drawing  in  this  part  of  the  nebula  is  more  limited  than  his, 
we  cannot  make  a  comparison  throughout  the  whole  extent.  This  confirms,  how- 
ever, the  exactness  of  our  figure  in  these  parts." 

OBSERVATIONS  OF  D' ARREST  (1872). 
"  ON  THE  NEBULA  IN  ORION  AND  ITS  SPECTRUM. 

"  BY  PROFESSOR  D'ARRE.ST,  1872. 
"[Translated  from  the  Danish  by  Dr.  WILLIAM  DOBKRCK.] 

What  follows  was  kindly  communicated  to  me  by  Dr.  DOBEREK,  and  it  has  been 
slightly  condensed  through  the  care  of  Miss  ELIZABETH  HARRIS,  who  is  familiar  with 
this  nebula  from  the  assistance  rendered  by  her  to  GEORGE  BOND  during  his  director- 
ship of  the  Harvard  College  Observatory.  It  is  given  here  almost  in  full,  as  in  its 
original  form  it  is  not  generally  accessible  to  English-speaking  astronomers. 

"CHAPTER  1 1. 

"The  brightest  and  most  interesting  part  of  the  nebula,  HUYGIIENS'  region,  with 
its  environs,  is  represented  on  the  plate  at  the  end  of  the  work,  and  may  be  consid- 


FIG.  35.     D'ARREST,  1872. 

ered  as  the  result  ot  my  observations  in  the  winters  from   1865  to   1871.     It  may 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  99 

especially  be  compared  to  the  representation  given  thirty-five  years  ago  by  M.  LAMONT,* 
as  the  large  refractors  in  the  observatories  of  Munich  and  Copenhagen  may  be  con- 
sidered as  perfectly  identical  in  optical  respects ;  and  we  have  not,  to  my  knowledge, 
hitherto  possessed  two  drawings  of  the  ^da-nebula  made  with  exactly  similar  instru- 
ments after  a  considerable  interval,  although  not  a  few  valuable  drawings  have  been 
made  within  this  interval.  While  plates  published  by  LASSELL  and  Lord  ROSSE  rep- 
resent the  details  of  the  nebula  as  seen  about  1853  and  1864  in  the  three  largest 
reflectors  which  have  ever  been  directed  to  the  heavens,  SECCHI'S  drawing  of  1865  was 
made  with  a  Q-inch  refractor,  and  GEORGE  BOND'S,  from  1857  to  1865,  with  a  1 2-inch 
object-glass  [14  Paris  inches.— E.  S.  H.].  The  appearance  of  the  nebula  is  known  to 
depend  in  a  considerable  degree  upon  the  optical  power  of  the  instrument.  The  first 
two  named  telescopes,  especially  that  of  Lord  ROSSE,  surpass  surprisingly  in  their 
effects  on  this  field  all  existing  refractors ;  older  contemporary  ones  are  consequently 
strictly  comparable  with  LASSELL'S  and  ROSSE'S  only  when  it  is  certain  what  alterations 
have  occurred  in  the  ^efa-nebula,  and  in  what  regions  they  have  occurred.f 

"  To  my  graphical  representation  I  have  added  a  general  view,  which  gives  the 
necessary  information  on  the  nomenclature  and  designations  used  at  present.  There 
was  no  occasion  to  introduce  new  names  in  the  region  referred  to ;  those  now  used 
arise  all  from  Sir  JOHN  HERSCHEL,J  0.  STROVE,  and  ROSSE.  We  shall  anon  speak 
about  certain  parts  previously  discussed,  for  instance,  Hemicyclmm  Liapunovii,  which 
we  are  no  longer  able  to  recognize  under  the  slow  variations  of  light  which  doubtless 
take  place  in  the  nebula. 

"A  comparison  between  the  two  perfectly  adequate  representations  of  the  central 
part  (LAMONT'S  and  my  own)  shows  that  HUYGHENS'  region,  in  conformity  with  what 
else  is  known  for  certain,  has,  on  the  whole,  not  materially  altered  its  form  and  appear- 
ance. The  separation,  however,  in  the  southwestern  part  between  the  forms  a,  ft,  x 
[E,  I,  F],  which  is  but  feebly  and  indistinctly  indicated,  appears  so  much  more  dis- 
tinct that  it  is  hardly  possible  that  the  divisions  can  have  presented  thirty- five  years 
ago  the  same  sharp  and  certain  outline  for  which  they  are  at  present  remarkable- 
This  suspicion  is  indeed  confirmed  by  HERSCHEL'S  first  drawing  of  1824;  but  in  the 
Cape  observations,  Vhere  certainly  the  nebula  was  seen  under  favorable  circumstances, 
there  appear  such  definite  traces  of  the  existing  main  separations  that  we,  on  the  other 
hand,  dare  not  assign  their  first  origin  to  so  late  an  epoch.  The  attention  of  astron- 
omers will  therefore  be  directed  to  this  point  in  the  [immediate  future]. 

a  In  LAMONT'S  figure  there  is  no  trace  of  the  strong  condensation  in  the  north- 
western corner  of  the  great  body,  about  the  place  where  STROVE  put  his  lien*  Secchii  : 
an  object  which,  however,  I  have  not  been  able  to  identify  from  the  description.^  I  have 

"  *  LAMONT.—  Uebcr  die  Ncbelflecken  (Academisctif  Schrijt)  Miiuchen,  1837.     JRegio  Hugeniana,  Fig.  XI. 

"  t  This  opinion  is  shared  by  O.  STRUVE,  the  most  competent  authority  in  this  respect;  compare  :  Vlert.  Jahr.  d. 
astr.  Gesellnch.  V,  page  26.  Jan.,  1870. 

"t  Memoirs  R.  A.  S.,  II,  page  490,  ei  seq.,  and  plate  VIII.  STRUVK  and  Lord  ROSSE,  in  the  works  mentioned  In 
the  first  section.  The  star-numbers  on  my  general  view  are  given  from  O.  STIU'VE'S  catalogue  in  the  paper  on  the 
nebula  in  Orion  (1862). 

"  §  I  have  repeatedly,  after  STRUVE'S  description,  and  according  to  his  measured  positions  and 

*  70  and  *  c,  sought  for  this  black  circular  spot  of  15"  diameter.  I  never  succeeded  in  finding  this  little  dark  opening, 
although  lacus  Secchii  no  doubt  was  to  be  found  in  October,  1857.  Everything  lion-  points  to  a  permanent  alteration  in 
the  distribution  of  luminous  matter.  I  do  not  know  that  any  other  astronomer  h:is  ten*  s«  ,-n  this  Secchtan  f.alr. 


JOO  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

every  reason  to  consider  my  own  representation  as  trustworthy.  As  I  am  on  this 
point  in  relatively  good  agreement  with  Gr.  BOND,  no  doubt  can  prevail  that  a  substantial 
alteration  has  occurred  here ;  so  much  the  less,  as  the  present  sharply  appearing  and 
precise  bounding  is  also  altogether  wanting  on  HERSCHEL'S  drawings  of  1824  and  1835. 

"Among  the  most  extraordinary  differences  between  LAMONT'S  and  my  representa- 
tion, I  further  class  ' pons  Schroeteri'  in  Sinus  magnus,  which  is  often  found  mentioned 
in  older  reports.  LAMONT  has  nothing  of  this  bridge  across  the  gulf  but  the  small 
[base],  which,  like  a  promontory,  is  attached  to  the  north  side  of  the  gulf,  while  on 
my  drawing  is  to  be  seen  a  perfect  communication,  with  two  brighter  points  about 
midway.  This  remarkable  difference  is  in  this  instance  but  a  corroboration  of  a  partial 
transformation,  or  rather  of  local  alterations  in  brightness,  which  were  pointed  out  as 
certain  by  STRUVE  as  early  as  1862.* 

"  LAMONT  has  hardly  a  recognizable  trace  of  the  two  perfectly  sure  and  thereby 
very  characteristic  configurations  on  the  west  side  of  Sinus  Gentilii,  round  the  stars  50 
and  54  [558,  573]  in  my  drawing,  which  are  almost  identical  with  those  of  BOND,  and 
almost  perfectly  identical  with  those  on  ROSSE'S  splendid  map.  Sir  J.  HERscHKLf 
remarked  long  ago,  and  no  doubt  justly,  that  the  outline  of  LE  GENTIL'S  gulf  was  not, 
on  the  whole,  correctly  given  in  LAMONT'S  diagram.  I  suppose  that  it  was  not  at  all 
LAMONT'S  intention  to  give  the  outer  parts  of  the  nebula. 

"Huyghens1  region  of  the  nebula  in  Orion,  apart  from  the  mentioned,  most  prominent 
differences,  is,  on  the  whole,  seen  far  more  finished  and  with  far  finer  particularities  in 
my  refractor,  at  Copenhagen,  than  might  be  presumed  from  the  drawing  made  in 
Munich.  LAMONT,  I  suppose,  did  not  at  that  time  make  the  large  nebula  an  object  of 
special  study.  We,  therefore,  need  not  attribute  great  weight  to  the  really  great 
difference  existing  between  these  two  drawings,  which,  with  an  interval  of  so  many 
years,  have  been  made  with  equally  excellent  instruments.  It  is  moreover  to  be 
remarked,  that  LAMONT  fills  the  inner  space  of  the  trapezium  with  as  dense  a  luminous 
matter  as  surrounds  it  on  all  sides.  I  see,  on  the  contrary,  the  six  trapezium  stars 
always  on  a  far  feebler  and  almost  dark  background.  HIERONYMUS  SCHROETER{  saw 
it  in  his  time  as  LAMONT  has  shown  it.  The  empty  space,  in  reality,  is  but  apparent : 
of  this  part,  however,  1  will  speak  at  length  in  §  14. 

"CHAPTER  12. 

"  I  shall  in  this  chapter  compare,  in  certain  points,  my  own  drawing  of  Huygliens'1 
region  with  other  lately  published  representations.  From  such  a  comparison  of  con- 
temporary drawings  made  by  the  aid  of  different  instruments,  elements  are  obtained 
which  will  in  future  ages  be  of  great  value.  However  great  the  difference  in  the 
whole  appearance  of  so  complicated  and  difficult  an  object  which  climatical  circum- 
stances may  produce,  the  artistic  representations  over  which  the  observers  have  but 
little  power,  may  very  easily  happen  to  exercise  a  far  greater  and  sometimes  disastrous 
influence  on  the  representation.  Remarks  in  a  negative  direction  may,  from  these 
reasons,  be  not  without  importance. 

'•  *  Observations  dc  la  grange  nebuleuse,  etc.,  page  1 16. 

"  t  Results  of  Astron.  Observations,  1847,  §  69- 

"{  Aphroditographische  Fragmenle,  Helrastedt.  1796.     PI.  II. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  IOI 

"A.  The  drawings  which  depict  the  nebula  in  its  whole  extent  give  occasion  to  the 

ollowing  remarks:  According  to  my  general  knowledge  of  the  nebula  in   Orion, 

>SSELL'S  steel  engraving,*  compared  with  the  image  in  a  ten  and  a  half  inch  refractor, 

represents  the  single  parts  of  the  nebala  much  too  loosely,  and  the  portions  around 

the  trapezium  are  in  particular  anything  but  successful. 

'  The  considerable  nebulosity,  for  instance,  which  closes  Sinus  magnus  to  the  west, 
and  whose  brightest  part  (according  to  ROSSE  it  is  traversed  by  a  curvilinear,  narrow, 
and  dark  channel)  extends  toward  the  trapezium,  is  almost  entirely  wanting.  There- 
from arises  round  the  trapezium  a  vacuum  of  an  extent  which  surely  is  not  to  be 
found  in  the  sky.  Neither  is  the  north  side  of  the  *  large  gulf  at  all  naturally  given. 
Further,  it  is  decidedly  wrong  that  *  nebula  Mairanni'  should,  as  LASSELL  represents, 
surpass  all  other  parts  in  brightness ;  it  has,  and  in  reality  never  had,  more  than  the 
third  or  fourth  position  in  brightness.  We  may  expect  that  these  wants  and  disagree- 
ments in  the  engraving  have  been  removed  in  the  later  drawing,  which,  after  the  return 
of  the  distinguished  astronomer  from  his  second  sojourn  in  Malta,  has  been  presented 
to  the  Astronomical  Society  in  London. 

"B.  In  GEORGE  BOND'S  posthumous  work  on  the  great  nebula  is  to  be  found  that 
representation  which  in  every  respect  reproduces  both  its  general  appearance  and  its 
finest  particularities.  It  might  seem  desirable  that  the  central  part  had  been  given 
apart;  but  although  the  longest  diameter  of  Huyghens1  region  now  is  not  longer  than 
eleven  lines  in  the  picture,  still  all  details  are  presented  sufficiently  distinctly ;  while  the 
characteristic  appearance  of  the  object,  and  especially  the  singular  serenity  which 
there  reigns,  is  in  perfect  accordance  with  nature.  My  drawing  of  the  main  mass  does 
not,  in  any  important  respect,  deviate  from  BOND'S.  I  have,  with  our  somewhat  infe- 
rior refactor  in  point  of  light,  found,  after  all,  only  the  northern  outline  a  little  different. 
Less  and  separate  masses  in  the  pointed  southeastern  corner  appear,  too,  more  distinctly 
by  BOND  than  I  have  ever  been  able  to  discern  them.  I  consider,  for  instance,  after 
repeated  inspection  and  comparison,  my  own  representation  of  the  separate  mass  to  the 
east  of  palus  Bondii\  more  conformable  to  the  true  form.  EOSSE  also  finds  it  very 
nearly  as  I  have. 

"  C.  I  do  not  find  ANGELO  SECCHI'S  large  copper-plate  of  1 868  quite  successful.  Of 
the  disagreements,  I  shall  only  mention  the  following :  The  parts  a  and  ft  [E  and  I], 
of  the  main  mass,  whose  real  figures  are,  of  course,  for  the  present  placed  altogether 
beyond  doubt,  are  hardly  to  be  recognized,  a.  [E]  has  really  four  sides  and  is  almost  a 
square ;  /?  [I]  has  decidedly  the  form  of  a  lengthened  triangle ;  but  in  the  Roman  repre- 
sentation they  are  both  shown  as  round,  almost  circular.  In  the  net  of  channels  which 
traverse  this  region  the  courses  are  too  broad.  To  the  west  of  Sin  its  Gentilii  all 
agreement  with  the  sky  is  wanting.  I  find  it  is  also  difficult  to  understand  how  the 

"  *  Memoirs  R.  A.  S.,  vol.  xxiii,  pi.  i. 

"  1 1  have  during  a  long  time,  used  this  Strnvian  denomination  (Observations,  etc  ,  page  n 

channel  (or,  according  to  ROSSE,  the  deep  inlet),  which  separates  the  luminous  isles  around  stars  50  and  54  S.  F.  (558, 
S70      But  it  was  perhaps  STRUVE'S  design  to  use  the  denomination  '  pa  I  us  Bondii'  for  th,-  .•a.sfn,  .sh-  ulom- :  in  tl 
case  the  words  'a  narrow  bridge'  does  not  well  correspond  to  existing  relations.     In  my  general  view  I  have  1 
the  latter  supposition.  " 


102  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

very  remarkable  radiations,  especially  those  which  emanate  from  the  south  side  of  the 
central  part,  can  have  escaped  the  notice  of  this  experienced  astronomer.* 

"No  doubt  much  time,  assiduity,  and  care  have  been  spent  in  the  Collegio  Romano 
'in  the  study  of  the  nebula  in  Orion;  the  whole  paper  and  some  single  observations  of 
fine  particularities  testify  this.  The  cause  of  the  disturbance  which  apparently  pre- 
vails in  the  drawing,  and  of  the  disagreement  which  exists  on  several  points,  I  attribute 
to  the  circumstance  that  SECCHI  made  use  of  moonlight  nights.  The  brighter  and 
brightest  parts  have  in  consequence  too  much  ascendancy  over  the  fainter,  and  the 
general  impression  has  thereby  become  somewhat  different  from  that  to  which  per- 
fectly dark  nights  have  accustomed  us. 

"  D.  As  far  as  I,  after  all,  can  be  in  possession  of  a  well-founded  opinion  on  the 
importance  of  the  large  and  splendid  drawing  which  is  the  main  result  of  the  work  of 
several  astronomers  during  many  years,  with  Lord  ROSSE'S  transcendant  reflector,  I 
shall  express  that  I  have,  by  degrees,  arrived  at  the  conviction  that  his  representation 
approaches  in  every  important  respect  most  nearly  to  the  true  state  of  the  nebula  as  it 
was  about  i862.f 

"  I  have  generally,  allowing  for  the  very  inferior  instrument  (in  point  of  light)  of 
this  observatory,  at  least  found  the  representation  by  Lord  ROSSE  to  agree  with  the 
truth.  This  verification  is  not  without  considerable  importance  for,  to  be  brief,  the 
refractor  surpasses  HERSCHEL'S  2O-foot  telescopes.  Only  in  feebler  extensions,  in  the 
very  faint  connections,  and  in  the  singularly  intertwined  bands  which  continue  the 
nebula,  especially  to  the  west,  I  sometimes  found  it  impossible  to  follow  traces  in  ROSSE'S 
drawing.  Furthermore,  I  share  the  opinion  that  the  outlines,  particularly  in  the  sepa- 
rate plate  of  Huygliend  region  (Plate  I)  are  sharper,  and  the  dark  furrows  somewhat 
broader  than  they  ought  to  be ;  that  the  contrast  between  the  brighter  and  fainter  parts 
are  very  strongly  marked,  at  least  when  we  judge  according  to  the  image  in  the 
refractor  of  Copenhagen. 

"I  shall  add  in  particular,  with  respect  to  regio  Huygheniana  and  parts  surrounding 
it,  the  following  remarks,  after  a  comparison  of  the  different  drawings  inter  se,  and 
with  my  own  observations. 

"  On  the  north  side,  I  never  saw  the  two  large  dark  bays  just  below  the  tra- 
pezium [W1  and  W3  of  Index-Chart]  appear  as  sharply  and  distinctly  as  on  ROSSE'S 
drawing.  In  LIAPONOFF'S  they  are  totally  wanting,  and  in  BOND'S  and  SECCHI'S  but 
barely  visible.  They  are,  in  reality,  but  darker  intervals  between  two  long,  bent,  tail- 
like  areas,  which,  trailing  off  to  the  west  and  northwest,  are  lost  far  away  between  regio 
Derhamiana  and  regio  Picardiana. 

"The  east  corner  of  the  main  mass  [B  in  SECCHI'S  diagram,  Plate  II,  A  in  ROSSE'S 
zr  Q]  is-certainly  bent  somewhat  upwards,  and  does  not  smoothly  pass  into  the  origin  of 

"*  I  had  some  years  ago  occasion  to  lay  stress  upon  certain  disagreements  of  a  similar  nature  (Astr.  Nachr.,  vol. 
Ixx,  No  1678,  page  342),  which  were  afterwards  explained  by  '  che  la  figura  litografica  publicata  henche  esatto  in 
generale,  ha  alcune  inesattesse  non  trasciorabili.'  (Sulla  grantle  ntbulosa,  page  27.)  The  possibility  of  such  an  expla- 
nation is  not  open  in  this  instance,  as  he  remarks  about  the  nebula  in  Orion,  '  cosi  siamo  sicuri  che  1'incisione  rappre- 
senta  la  nebiilosa  come  vedesi  da  noi  uel  nostro  strumento.' 

"  1 1  refer  to  the  privately  distributed  plate  which  represents  the  object  on  a  black  ground,  and  which,  no  doubt, 
is  also  in  technical  respects,  one  of  the  most  excellent  productions  of  art.  This  'plate  is  not  annexed  to  the  paper 
in  Phil.  Tram.,  but,  on  the  contrary,  another  one,  executed  in  the  common  way.  Volume  for  1868. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  iO" 

•  O 

proboscis  major ;  ROSSE'S  words,   '  curving  slightly  in  opposite  directions',  convey  the 
best  idea  of  the  way  in  which  the  transition  takes  place. 

"  I  have  remarked  above  that  I  unconditionally  adhere  to  my  own  conception  of 
the  parts  beyond  the  west  side  of  Sinus  GentUii,  which  approaches  essentially  to 
ROSSE'S  image,  and  is  not  contrary  to  BOND'S.  But  with  this  conception,  the  traces  to 
which  LIAPONOFF'S  diagram  on  this  point  confines  itself  do  not  well  agree.  The  bright 
and  rather  shapeless  luminous  masses  which,  according  to  SECCHI,  between  K.  L.  12 
[i.  e,,  between  /*  and  6  of  Index-Chart],  on  the  Roman  plate,  seem  to  rival  in  bright- 
ness the  most  apparent  parts  of  HUYGHENS'  region,  did  not  exist  during  the  time  I 
observed.  [These  are  probably  the  masses  attached  to  the  north  shore  of  //.] 

"Sinus  Lamontii  I  almost  invariably  found  filled  with  rather  dense  nebulosity; 
here,  however,  the  relative  brightnesses  seem  so  very  inconstant  that  single  objects 
are  sometimes  with  difficulty  recognized.  In  the  adjacent  Hemicyclium  Liapunovii 
greater  changes  in  brightness  have  doubtless  occurred  since  O.  STRUVE'S  investigations 
in  1861  ;  but  on  this  point  even  later  contemporary  representations  agree  but  badly. 
I  should  like  to  know  if  the  luminous  heap  which,  according  to  SECCHI,  extends  in 
m.  %  n.  from  9^  to  io>£,  may  be  considered  as  part  of  the  Hemicyclium. 

"An  agreement  in  this  domain  seldom  to  be  found  occurs  with  respect  to  the  large, 
very  deeply  indented  bay  which  0.  STRUVE  has  called  '  Locus  Lassellii?  Its  outline 
is  still  exactly  as  it  was  given  for  1857:  stars  76,  80,  and  84  (652,  657,  and  663),  a 
little  outside  star  89  (669)  somewhat  inside  the  nebula.*  SCHROETER  observed  this 
remarkable  indentation,  or  rather  intersection,  as  early  as  1 795  and  1 799.  Notwith- 
standing possible  variations  in  brightness,  it  can  be  proved  that  here  during  the  last 
seventy  years  no  variation  whatever  in  form  has  taken  place.f 

"It  is  on  the  whole  this  constancy  of  form  which  I  consider  the  most  important 
result  of  the  whole  study  which  has  been  spent  on  the  nebula  in  Orion.  The  observed 
variations  in  this  extensive,  gaseous  mass  seem  solely  and  exclusively  to  end  in  tem- 
porary, luminous  fluctuations,  particularly  in  certain  regions.  Generally,  perhaps 
always,  the  old  forms  reappear  after  a  shorter  or  longer  time.  I  have,  during  a  lapse 
of  years,  seen  instanses  of  this  in  pom  Schroeteri,  Sinus  Lamontii,  and  Sinus  Lassellii. 
Fluctuations  of  light,  which,  taken  apart,  are  seemingly  inconsiderable,  produce  here 
sometimes  remarkable  alterations.  Several  years  ago,  when  examining  a  representa- 
tion dated  1779,  in  the  22d  volume  of  ROZIER'S  'Observations  de  la  physique,'  I  found 
the  following,  which  leads  to  the  same  result :  '  That  we  in  the  outline  of  the  theta- 
nebula  are  able  to  point  out  an  invariability  in  form  which,  the  nature  of  the  object 
considered,  is  plainly  surprising.'  At  that  time,  spectroscopical  investigations  which 
have  assured  us  in  regard  to  the  physical  constitution,  were  yet  unknown  to  me." 

"*  STRUVE  :  Observations,  etc.,  page  103. 

"  tBeitriige  z.  den  ueuesten  Astr.  Entdeck.,  Ill  vol.,  Gottingen  1800,  page  231,  and  figure  39,  Table  V,  to  COOL 
pare  with  Aphroditographische  Fragraeute,  page  247.    SCHKOETKR'S  star  near  (9)  must  be  either  76  or  80  of  STI; 
catalogue.     The  passage  in  vol.   II   of  Melanges  Math,   et  Astr.,  St.   Petersburg,    1854,   page  531,  where  i 
about  this  channel  that  "it  never  was  represented  by  any  other  astronomer"  is  hardly  correct. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


OBSERVATIONS  OF  WLNLOCK  AND  TROIJVELOT  (1874). 

Fig.  37  is  a  copy  of  a  pastel 
drawing  made  in  1875.  It  was 
published  in  NEWCOMB'S  Popular 
Astronomy,  page  446,  Fig.  104. 
The  electrotype  kindly  given 
me  by  the  publishers  of  this 
work  has  been  mislaid  at  the 
Naval  Observatory,  and,  owing 
to  my  absence  from  Washington, 
it  cannot  now  ( 1 882,  January  3 1 ) 
be  found.  I  refer  the  reader  to 
the  cut  in  Professor  NEWCOMB'S 
work. 


FIG.  36.     WINLOCK  AND  TROUVELOT,  1874. 


FIG.  37.    TROUVELOT,  1875. 


The  drawing  of  the  central  portion  of  the  nebula  of  Orion  given  in  Fig.  36  was 
made  by  M.  L.  TROUVELOT  at  the  observatory  of  Harvard  College  in  1874,  under  the 
direction  of  the  late  Prof.  JOSEPH  WINLOCK. 

The  Fig.   37  represents  the  sketch  of  the  nebula  of  Orion  made  by  M.  TROU 
VELOT   with   the    26-inch   refractor  of  the   Naval   Observatory  of  Washington.     It 
is  avowedly  but  a  sketch,  but  is  of  value  for  comparison.     A  poor  representation  was 
published  by  the  heliotype  process  in  the  Washington  Observations  for  1874,  Appendix 
I.     Fig.  37  is  copied  from  Fig.  104,  p.  446  of  NEWCOMB'S  Popular  Astronomy. 

OBSERVATIONS  OF  DOBERCK  (iSyy-'yS). 

In  the  Astronomische  Nachrichten,  .vol.  91,  col.  335,  Dr.  DOBERCK,  in  an  article 
entitled  "  Remarks  on  Nebulae,"  refers  to  the  nebula  of  Orion,  as  follows: 

"Mr.  COOPER  made  also  a  drawing  of  the  OHow-nebula  [this  drawing  has  been 
previously  described],  and  by  comparing  that  with  the  image  of  the  nebula  seen  last 
year  (1877)  in  the  indentical  refractor,  I  had  a  rare  opportunity  of  corroborating  the 
changes  which  D' ARREST  has  pointed  out."  *****  Then  follows  the  order 
of  brightness  of  the  various  parts.  The  brightest  part  of  the  Orww-nebula  is  the 
northwest  corner  of  the  central  part  [D],  then  [A],  then  Q,  then  I  and  E. 

V  as  about  as  bright  as  /u. 

This  is  all  that  immediately  relates  to  the  Huyglienian  region,  but  the  complete  article 
should  be  consulted,  as  it  is  a  record  of  the  order  of  brightness  of  the  wlwle  nebula. 

OBSERVATIONS  OF  TROUVELOT  (1876?). 

M.  TROUVELOT  kindly  undertook  to  make  some  observations  on  the  nebula  with 
different  apertures  and  eye-pieces  according  to  a  scheme  which  I  submitted  to  him. 
Unfortunately  the  notes  made  by  him  he  has  not  been  able  to  find,  and  the  following 
memoranda  of  the  intensity  of  the  light  in  various  portions  of  the  nebula,  as  marked 
on  .a  copy  of  the  Index-Chart,  are  all  that  now  rSmain. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  1O5 

k        He  noted  on  the  Index-Chart  the  brightness  of  various  portions  on  a  scale  of  i  to  4 

1  was  "  brilliant." 

2  was  "  half- tint." 

3  was  "quarter-tint." 

4  was  "black." 

In  what  follows  I  have  given  his  results  in  my  own  words  from  the  marks  on  the 
chart. 

rf  (near  frons),  3. 

Z  (half-way  from  Sinus  G.  to  581),  3. 

/?  (near  the  letter  ft  on  chart),  2. 

K  (near  the  letter  K  on  chart),  2. 

7,  4- 

3  (near  figure  3  on  chart),  3. 

Channel  between  if>  and  4  (half  way  from  letter  iff  on  chart  to  letter  W),  2. 

[This  is  my  mass  W2.] 
Channel  between  if)  and  Xj  o- 
W3,  3- 

W4,    2. 

A,  i. 

D,  i. 

q>  (half  way  from  star  2  to  star  652),  2. 

Lacus  Lassettii  (near  star  3),  3. 

r',4- 

f,4. 

ore  (1/3  the  way  from  o  to  TT\  3. 

ff  (y±  the  way  from  o  on  chart  to  681),  2. 

P,  3- 

follnwing  p,  4. 

//,  2. 

Half  way  between  708  and  741,  and  20"  north  of  the  line,  3. 

Dark  band  between  x  and  A  and  near  666,  3. 

OBSERVATIONS  OF  LANGLEY  (1879). 

Professor  LANGLEY  visited  Mount  Etna  in  January,  1879,  taking  with  him  a  small 
CLARK  equatorial  of  3^  inches  aperture,  and  making  observations  of  various  kinds  for 
the  purpose  of  determining  the  effect  of  high  altitudes  upon  telescopic  vision.  This 
expedition  was  made  under  the  auspices  of  the  United  States  Coast  Survey,  and  I  am 
indebted  to  the  Superintendent  of  the  Survey  and  to  Professor  LANGLEY  for  the  ready 
permission  to  publish  the  interesting  and  very  valuable  drawing  shown  in  Fig.  38, 
together  with  the  notes  accompanying  it.  It  is  to  be  noticed  that  Professor  LANGLEY 
APP  v 14 


fo6  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

has  successfully  applied  to  the  nebula  of  Orion  the  method  of  contour  lines  imagined 
and  first  used  by  MASON  in  his  drawing  of  the  trifid  nebula  in  1837. 


FIG.  38.     LANGLEY,  1879. 

"ALLEGHENY  OBSERVATORY, 

"Allegheny,  Pennsylvania,  April  27,  1880. 
******  * 

"  I  send  by  this  two  faithful  copies  of  my  original  sketches  of  Orion  (nebula)  made 
on  Etna  last  year  with  your  Naval  Observatory  telescope  of  3  ^  inches  aperture.  They 
were  made  in  haste,  in  intervals  of  other  work  which  took  up  nearly  all  the  few 
clear  hours.  I  think  their  value  (if  any)  lies  in  the  fact  that  the  person  who  made 
them,  while  having  some  little  experience  in  such  sketches,  was  by  chance  almost 
absolutely  ignorant  of  the  aspect  of  the  nebula  in  large  instruments  (I  have  not  looked 
at  it  for  many  years),  and  did  not  at  all  know  what  he  ought  to  see.  There  were  two 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  1O; 

or  three  rough  sketches,  of  which  the  India  ink  one  here  [omitted]  is  the  summary,  so 
it  does  not  very  closely  agree  with  the  outline  [Fig.  38],  where  the  contours  and 
inclosures  are  marked  from  i  +  (brightest)  to  7  (faintest).  This  was  chiefly  done  on 
one  evening  by  beginning  in  the  earliest  twilight  when  only  a  little  haziness  was  visible 
near  0,  and  sketching  new  contours  every  10  minutes  or  so  as  the  sky  grew  darker. 
I  have  scrupulously  abstained,  even  in  making  the  copies,  from  reference  to  any  pub- 
lished drawing. 

*  *  *  *  *  *  * 

"S.  P.  LANGLEY." 

We  may  compare  this  photometric  sketch  of  the  nebula  with  the  previous  draw- 
ings, as  follows:  HUYGHENS  (1656),  Fig.  i,  differs  largely  from  LANGLEY'S  outline  i  ; 
the  trapezium  and  685,  708,  741  are  involved  in  nebula  in  1656,  and  not  so  later. 
The  region  near  the  north  shore  of  Sinus  magnus  was  seen  much  farther  by  HUYGHENS 
than  light  of  the  same  intensity  according  to  LANGLEY.  The  reverse  of  this  is  true 
just  preceding  the  trapezium.  HUYGHENS'  drawing  of  1694  (Fig.  2)  is  undoubtedly  a 
better  representation  of  the  appearance  of  the  nebula  in  his  time  than  the  earlier  one. 
Comparing  this  with  LANGLEY  we  find  a  very  good  agreement  with  his  outlines  1 1 1  .  .  , 
with  the  same  exceptions  as  before;  i.  e.,  HUYGHENS'  region  near  a  is  brighter  than 
LANGLEY'S  and  his  trapezium  is  within  the  nebula. 

It  is  the  same  in  MAIRAN'S  Fig.  3   (1731),  and  also  in  LONG'S  (1742)  Fig.  5 
PICAKD  (1673)  Fig.  4,  agrees  better  with  L  \NGLEY  when  note  is  taken  of  the  different 
kinds  of  shading  than   when  the  simple  outline  is  taken,  and  this  seems  to  be  an 
important  point 

LE  GENTIL  (1758),  Fig.  6,  seems  to  have  seen  out  to  LANGLEY'S  222  .  .  .  and 
towards  the  north  preceding  portions  even  as  far  as  333  ....  Here  again  the  portion 
o  is  brighter  in  the  older  drawing  than  in  LANGLEY'S. 

MESSIER  (1771),  Fig.  10,  agrees  much  better  with  LANGLEY  than  any  of  the  pre- 
ceding ;  a  comparison  of  the  two  figures  should  be  made ;  MESSIER'S  o  is  very  much 
as  drawn  by  LANGLEY.  He  seems  to  have  seen  out  to  LANGLEY'S  444  .  .  .  LEFEBVRE 
(i  779),  Fig.  13,  saw  out  to  LANGLEY'S  333  .  ...  on  the  following  side,  but  hardly  so  far 
just  west  of  the  trapezium.  However,  his  figure  is  grossly  misdrawn.  In  SCHROETEB 
(1794),  Fig.  14,  the  neighborhood  of  the  trapezium  is  very  different,  the  southern  end 
of  E  is,  as  before  remarked,  strangely  so. 

Figs.  29  and  30  should  be  compared  with  Professor  LANGLEY'S,  as  well  as  Fig.  12, 
and  the  description  by  DOBERCK  in  Astronomische  Nachrichten,  band  xci,  col.  336, 
No.  2 1 8.  The  earlier  figures  all  seem  to  give  a  greater  brightness  to  the  region  near 
6  than  Professor  LANGLEY'S  outlines  warrant.  On  the  whole  I  am  inclined  to  regard 
this  as  accidental. 

An  important  paper,  by  my  friend  Mr.  KNOBEL  (Monthly  Notice*  R.  A.  S.,  vol. 
41,  p.  312),  gives  the  results  of  his  photometric  measures  on  the  relative  brilliancy  of 
three  portions  of  this  nebula.  Unfortunately  for  my  purpose  different  parts  of  the 
Huyglienian  region  are  not  compared  in  such  a  way  as  to  assist  in  the  photometry  of 
the  various  masses  laid  down  in  the  Index-Chart. 


IOS  MONOGRAPH   OF  THE  CENTRAL  PARTS  OE  THE  NEBULA  OF  ORION. 

PAET  II.— WASHINGTON  OBSERVATIONS  OP  THE  NEBULA  OF  ORION,  IN  CHRONO- 
LOGICAL ORDER.* 

1874,  JANUARY  n. 

i  ih  30™ — i2h  i5m.  Companion  to  724  not  seen.  [This  companion  is  laid  down  in 
LASSELL'S  chart,  Mem.  R.  A.  S.,  vol.  xxiii  (1854),  and  was  discovered  independently  by 
ALVAN  G.  CLARK,  with  the  i8£-inch  refractor,  at  Chicago  in  1862.  It  is  not  in  G.  P. 
BOND'S  Catalogue.  It  is  mentioned  as  a  new  star  by  TISSERAND  ;  Bull.  Int.  Obs.  Paris, 
1876,  No.  1 19,  and  Comptes  Eendus,  Ixxxi,  p.  891.] 

1874,  JANUARY  14. 

9h.  Mag.  power,  400 :  very  good  seeing  at  times.  Companion  to  724  seen  neatly. 
North  of  635  and  641  saw  at  times  quite  plainly  (i)  [see  Index-Chart];  suspected 
strongly  a  star  at  (2)  ;  saw  twice  or  thrice  a  point  of  light  at  (3)  ;  saw  a  companion 
to  707,  (4).  I  could  see  no  stars  inside  the  trapezium. 

1874,  JANUARY  16. 

7h  to  9h  30™.  Eye-piece  400 :  seeing  poor ;  stars  bright  but  unsteady.  In  tha 
neighborhood  of  635,  etc.,  I  see  only  BOND'S  stars.  (641)  [one  of  0.  2,'s  variables]  very 
faint.  675  not  seen,  but  in  looking  for  (4)  I  could  just  occasionally  see  the  compan- 
ion to  724,  (a)  obliquely,  but  never  by  direct  vision,  although  I  tried  repeatedly.  //.  78 
(•=.  G.  P.  B.  (654))  seen  at  7h  and  until  7b  2Om  (approximately),  but  not  after  7h  30™. 
9h~9h  3°m>  tne  seeing  is  rather  worse. 

1874,  JANUARY  17. 

9h.  Eye-piece  400:  seeing  excellent.  Nothing  new  near  635.  612  seen  for  the 
second  time  double  [i.  e.,  618  also  seen  well].  (567)  very  faint.  (642;  not  seen^ 
although  its  two  neighbors  [647  and  651]  are  well  seen.  675  not  seen  ;  I  have  never 
seen  it.  (a)  [near  724]  seen  well.  (575)  seen.  (602)  seen  faint.  Besides  the  stars 
mentioned  I  see  near  the  trapezium  612,  618,  601,  621,  and  636,  the  last  very  faint. 

1874,  JANUARY  23. 

Many  and  thick  clouds.  (641)  seen  once.  Suspected  h.  78  —  (654),  but  too 
cloudy  to  verify.  Seeing  fair.  End  ioh  30™. 

1874,  JANUARY  24. 

9h  30™.  Prof.  C.  A.  YOUNG  examined  the  neighborhood  of  635,  669,  etc.,  with  eye- 
piece 400  [negative].  He  put  in  on  a  sketch-map  my  stars  (i),  (2),  and  (3)  of  Jan- 
uary 14  without  knowing  of  my  observation  of  that  date.  («)  he  sees  well.  Neither 
of  us  see  my  (4)  [near  707].  675  seen  by  both  of  us.  It  is  very  faint.  Seeing  occa- 
sionally fine.  Both  observers  say  there  are  no  stars  inside  trapezium.  No  trace  of  the 
resolvability  spoken  of  by  Lord  OXMANTOWN  [Phil.  Trans.,  1867]. 

*The  observations  are  recorded  as  written.  Additions  and  explanations  are  inserted  in  square  brackets,  []. 
The  observer  was  HOLDEN,  unless  otherwise  mentioned.  The  times  are  Washington  mean  times.  The  26-inch  telescope 
was  employed  with  its  full  aperture,  except  in  a  few  cases,  which  are  noted. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  109 

[The  stars  (i),  (2),  and  (3)  are  about  the  smallest  stars  that  can  be  seen  if 
immersed  in  nebulosity.] 

1874,  JANUARY  25. 

(i)  and  (3)  seen,  faint,  and  seen  only  at  times;  suspected  (2)  and  could  not  see 
(4).  Seeing  fair.  h.  78  —  (654),  (642),  and  675  not  seen,  (a)  seen. 

1874,  FEBRUARY  5. 

8h.  Haze :  stars  steady.  *  *  *  9*.  During  a  short  period  of  good  seeing  saw 
(641)  and  (i)  and  (3),  also  675.  This  last  is  extremely  difficult.  Is  709  variable? 
It  is  quite  faint  this  evening. 

1874,  FEBRUARY  14. 

Although  seeing  had  not  become  good  it  was  steady.  *  *  *  (654)  and  (602) 
seen.  Star  suspected  n.  f.  the  following  star  of  the  trapezium  and  not  far  distant.  LAS- 
SEI  L'S  b  [a  double  between  685  and  708]  looked  for  especially  and  not  found. 

1874,  FEBRUARY  17. 

737  yellow  and  dull  and  nebulous;  if  I  am  not  mistaken  in  the  number  (737)  it 
has  a  small  star  preceding  and  north  of  it. 

1875,  JANUARY  22. 

[Made  various  experiments  to  obtain  a  method  of  drawing  the  nebula  directly, 
i.  e.j  to  throw  its  image  on  a  surface  by  reflection  or  otherwise,  and  to  trace  it.  Among 
other  trials,  placed  a  piece  of  finely  ground  glass  in  principal  focus,  and  removing  the 
eye-piece,  I  saw  on  the  glass  the  trapezium,  635,  669,  685,  708,  and  741,  and  other 
stars  Made  a  sketch  on  the  glass.] 

1875,  OCTOBER  27. 

i2h.3  to  i3h.2.  Very  bad  seeing.  Win  i.  A  star  (575)  exactly  on  preceding 
edge  of  A.  which  is  very  faint  compared  to  following  edge. 

The  north  end  of  A  has  dark  channels  in  it  of  the  shape  figured  by  G.  P.  BOND 

[drawing  omitted]. 

Order  of  brightness. 

1.  A  (all  the  brighter  streaks  in  it1. 

2.  D  (following  edge). 

3.  F  =  Gr  (not  including  the  region  round  X  in  F). 

4.  L 

6.  H  =  E. 

[I  note  here  that  the  boundaries  of  N  and  Q  are  rather  uncertain,  under  ordinary 
conditions,  and  that  too  great  weight  must  not  be  given  to  comparisons  of  Q  and  N 
with  O  P  K,  etc.]  666  and  667  just  seen.  Suddenly  much  brighter,  just  north  of  647. 
647  >  (640  J  647  =  (670  ;  647  >  (575)  *  Seeing  very  bad.  [According  to  BOND, 
(647)  is  i2"\i  ;  (641)  is  i4m8;  671  is  1.1-5;  (575)  i»  >»I"-9-] 


I  10  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1875,  OCTOBER  29. 

Begin  i2h,  end  i3h.  Mag.  power,  175.  Wt.  —  2.  647  =  (575)  >  671,  but  not 
much.  (575)  >  573-  651  and  (654)  about  equal.  (641)  not  seen.  Reading  of  po- 
sition-circle for  parallel  =.  165°. 2. 

Frons. 

Micrometer  wire  placed  so  as  best  to  coincide  with  the  frons  (p.  estimated  n  40°), 
i  e ,  to  the  south  edges  of  E,  F,  N,  and  Q. 

Position-circle  readings :  24°. 8,  24°. 5,  2 7°. 5,  28°. 9,  28°. 8  ;  mean,  26°. 9.  Con- 
cluded position-angle,  48°. 3  (5)  (=  75°. 2  —  26°. 9). 

Occiput. 

Angle  of  position  (est.)  135°  ;  this  refers  to  preceding  edges  of  masses  E  and  I. 
Posi tion- circle :  ii2°.o,  112°. 2,  ii4°4,  H4°.4,  112°. 3;  mean,  113°.!  .'.p— 142. °i 
(5).     The  occiput  is  exactly  parallel  by  measure  to  a  line  joining  506  with  [570]. 

Position-angle  of  preceding  edges  of  J  and  B. 

Position-circle:  67°. 5,  65°. 9,  69°. 2  ;  mean,  67^5  .  - .  p  =  7°. 7  (3).  B  points  ex- 
actly to  575,  which  is  at  the  very  end  of  it.  The  reading  69°. 2  above  will  serve  to 
determine  the  angle  of  B  from  its  base  [i.  e.,  north  end]  up  to  575  .  • .  p  —  6°.o  (i). 

Order  of  brightness. 

i.  A.  2.  D.  3.  F  and  G.  4.  I.  5.  N  and  Q.  6.  E.  Perhaps  the  following 
is  better:  i.  A.  2.  F,  G,  D.  3.  I.  4.  H.  5.  N.  6.  Q  =  E.  The  general  effect  of 
each  mass  is  taken,  i.  e.,  the  brightest  parts  have  most  influence. 

1875,  NOVEMBER  5. 
i  ih. 5  to  i2h.5.  Mag.  power,  175.     Wt,  =  2. 

Lacus  Lassellii. 

652,  657,  663  are  on  the  preceding  side  of  a  dark  space  which  ends  a  little  to  the 
south  of  663  ;  they  are  inclined  across  it  from  s.f.  to  n.  p.,  652  being  nearest  the^re- 
ceding  side  of  the  space.  647  >  (575)  =  (671). 

V. 

The  ground  on  which  the  trapezium  stands  is  not  totally  black. 

r. 

The  north  part  (n.  two-thirds)  of  r  is  not  black  but  filled  with  nebulosity ;  the 
south  one-third  is  certainly  darker,  but  whether  totally  black  the  seeing  is  not  good 
enough  to  determine. 

Pons  Schroeteri 

In  the  middle  of  the  bridge  of  SCHROETER  the  nucleus  is  not  stellar,  *  *  *  at 
least  it  does  not  seem  to  be  so  to-night. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  T  i  i 

6. 

Along  the  south  edge  of  G  it  is  notably  brighter,  and  at  the  preceding  end  of  this 
terminating  bright  streak  I  think  I  see  a  small  star. 

a. 

The  Index-Chart  is  right  in  making  G  concave  towards  the  south.     [This  was 
marked  to  be  examined  again.     It  was  found  to  be  correct  1877,  December  12.] 


On  the  following  side  of  the  pons  it  is  quite  dark.  The  line  in  the  Index-Chart 
seems  to  limit  this  dark  space  properly. 

T". 
North  of  OTT  it  is  quite  dark. 

4?. 

To  the  south  of  OTT  it  is  filled  with  nebulosity. 

There  is  at  least  one  dark  streak  in  £  parallel  to  OTT  and  just  south  of  it  [con- 
trast?]: also  in  4;  I  think  I  see  a  star  half  way  between  it  and  the  Huygkenian  region, 
on  the  same  meridian  as  o. 

•        G. 

G  much  brighter  on  its  south  edge :  then  fainter  and  then  all  the  preceding  half 
is  brighter  than  the  following  half  (roughly  speaking). 

c. 

Branch  c  extending  to  523  is  about  right  in  Index-Chart;  perhaps  a  little  too 
much  curved.  It  runs  a  little  to  the  north  of  523  ;  but  that  star  is  nebulous,  at  least 
to-night. 

Order  of  brightness. 

i.  A.  2.  D.  3.  F  and  G.  4.  H  and  I.  5.  N  and  Q.  6.  E.  In  this,  however, 
only  the  brightest  parts  of  N  arid  Q  are  included. 

I  do  not  think  the  stars  685,  708,  741  have  cometic  tails  or  brushes  to  them 
extending  towards  the  south  as  BOND  notices.  The  nebulosity  seems  darker  between 
685  and  708  and  708  and  741  (in  the  parallel  nearly),  but  I  take  it  this  is  the  effect 
of  contrast  merely  [drawing  omitted].  It  is  darker  between  685  and  708  than  between 
708  and  741. 

1875,  NOVEMBER  10. 

Begin  nh  45"',  end  13''  20™.     Mag.  power,  175.     Wt.  =  2. 

Measures  of  Ad  with  &l  Orionis. 

Coincidence  of  fixed  wire  and  micrometer  wire  =  64'. 2 5.  Fixed  wire  on  6',  mi- 
crometer wire  on — 


I  12 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


Object  measured. 

Reading. 

A* 

Moving  mic. 
wire  south. 

Moving  mi- 
crom.  wire 
notth. 

Mean. 
Revolu- 
tions. 

Seconds  of 
arc. 

r. 

r.                     r. 

„ 

79-32 

78.98 
78.94 

j-        79-°8         -   14-83 

-I47-5 

refr.       o.i 

s« 

A<5—  147.6 

74-64 

74.16 

•63 

•34 

74-46         _   10.21 

—  101.6 

•  53 

. 

refr.    —o.i 

A  (5—  101.7 

Brightest  part  of  F,  on  the  same  par- 
allel as  685. 

74.16 

74-62 
.40 

74.39        —   10.14 

—  100.9 

refr.       o.  I 

A'5  —  101  .0 

70.63 

70.83 

70.73        —     6.48 

•  —  fi/i    e. 

U4+  •  5 

refr.       o  .  o 

Brightest    part    of    pans    Schro't,rii 
(whose  center  is  an  elongated  oval) 

60.43 

.25 

60.  15 

(•       60.28        +     3.97 

+  39-5 

on  same  parallel  as  bright  star  in 

.46 

60.  n 

) 

D  [=647]- 

refr.       o.o 

Bright  sharp  n./.  end  of  D     ... 

56.74 

56.15 
.44 

""  —  ;T;»y.5 
[        56.41        +     7-8i              +77-7 

refr.       o.o 

S.f.  sharp  point  of  a  =  "  Spitze" 

56.57 
•29 

56.40 
•23 

[        56.37        +     7-88              +78  4 

refr.       o  .  o 

Order  of  brightness. 


ist  A! 

2d     A 


Dl 

D 


FG  |  IH 
GF  |  IH 

.  • .  G  =  F. 


NQ 
NQ 


E. 
E. 


There  are  certainly  no  wisps  or  tails  to  685,  708,  and  741  (for  a  moment  seeing 
good).  Mag.  power,  400.  The  star  ROSSE  56  exists,  and  the  line  joining  it  and  581 
is  perpendicular  to  occiput. 

Sinus  Gentilii. 


If  there  is  any  totally  black  inlet  from  the  south  into  Sinus  Gentilii  it  is  very 
narrow.     The  Sinus  is  quite  black  in  its  north  end. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  113 

r. 

The  north  half  of  r  is  filled  with  light  which  join     n  to  pons  Schroeteri. 

W1  =  lacus  Secchii. 

The  distance  from  the  hole  (very  black)  Wt,  just  north  of  W,  to  685,  is  about 
equal  to  the  distance  from  635  to  669.  W2  is  north  of  it,  and  is  another  remarkably 
black  space.  Wl  and  W2  wrong  in  position  on  Index-Chart. 

1875,  NOVEMBER  1 1. 

i  ih  40m  to  1 2h  30m.     Mag.  power,  1 75.     Wt.  —  4. 

Occiput. 

Position- circle :   ii4°.o,  114°.  2,  U5°.9,  115°.  5.     Mean,  114°. 9.     .  • .  p  =  i39°.3. 
This  is  a  measure  of  the  preceding  edges  of  E  and  I,  and  it  cuts  Y  off  entirely. 

Frons. 

Position-circle:   22°.o,  22°. 3,  22°. i,  2i°.8.     Mean,  22°. r.     . '.^  —  52°. i. 
This  measure  is  the  best  tangent  to  the  whole  line  of  light,  but  it  cuts  off  some 
masses  at  s.  p.  corner  of  E,  and  some  at  Q. 

'  Q,  P,  R. 

Angle  of  position  of  n.f.  sides  of  Q,  P,  and  R  (estimated  100°) ;  the  line  passes 
through  654  and  O4,  or  nearly  so.  This  cuts  off  some  of  R  and  most  of  T. 

Position-circle:  328°. 2,  327°. 2  (good),  328°. 2,  329°.8.  Mean,  328°4.  .'.p  — 
io5°.8  (4). 

o. 

Angle  of  position  of  south  edge  of  G  and  general  north  shore  of  Sinus  maynus  up 
to  D  (to  the  north  bright  end  of  D).  [This  does  not  mean  that  D  was  one  of  the 
points  of  the  line  measured.]  The  measure  is  of  the  general  trend  of  the  shore. 

Angle  (est),  95°. 

Position-circle:  345°-O,  344°-8,  343°-4,  344°-7-    Mean,  344°-5-      '  •  /'  =  89°-7  (4)- 

(676)  =  (654)  =  or  >  (641). 

(654)  >  612  or  6 1 8. 

(654)  —  622  >  63 1.     ? ?  Query  62 1  1 

(631  is  in  a  black  space)      622  on  the  Index-Chart  is  properly  figured  as  to 

edges  of  A  and  U. 

P,  S,  M. 

The  south  edges  of  P  and  S  right  with  respect  to  67 1  and  676.  The  umih  >//«//•- 
•iHf/  edge  of  M  should  be  a  little  further  off  in  the  Index-Chart. 

PIHIS  Srhrocfrri. 

The  micrometer  wire  through  the  jww*  passes  through  685  and  669  nearly.  Par- 
allel, 344°-5- 

APP.  V 15 


114  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Position-circle:  84°.6,  82°.3,  83°.;.     Mean,  83°.  5.     .'  .p  —  170°.;  (3)- 
LASSELL'S  star  b  [a  double  between  685  and  708]  does  not  exist  [as  far  as  can 
be  seen  to-night]. 

685,  708,  741. 

It  might  be  said  that  there  is  a  wisp  or  tail  to  the  south  of  685  ;  between  685  and 
708  is  a  darker  space  [drawing1  omitted].  The  shaded  portions  [of  the  sketch]  repre- 
sent the  dark  spaces  of  which  685-708  is  much  the  darker.  This  may  be  the  effect 
of  contrast,  and  probably  is  between  708  and  741,  but  not  altogether  so  between  685 
and  708. 

Messierian  branch. 

The  preceding  edge  of  the  Brachium  Mess,  follows  784  and  789  as  in  drawing 
[omitted]  675  not  seen;  indeed  I  have  only  seen  it  once  or  twice  in  i873,-'74,-'75. 
686-688  not  seen,  but  not  carefully  looked  for.     R.  56  plainly  seen.     *     *     *     * 

A  and  L. 

In  the  south  part  of  A  and  the  north  part  of  L  the  shapes  are  not  caught  in  the 
lithograph.  It  looks  like  a  rope  with  the  strands  untwisted  so  that  you  may  see  be- 
tween them  There  are  at  least  two  such  openings. 

"Spitee"  in  (<?)  T,  T',  741. 

In  G.  P.  BOND'S  drawing  the  "  Spitze  "  on  south  shore  of  Sinus  magnus  is  too  bright 
[relatively],  but  not  much.  The  north  third  of  that  part  of  the  Sinus  magnus  pre- 
ceding the  pons  (T)  is  too  bright,  and  the  space  just  following  the  pons  is  also  too  bright. 
It  is  now  quite  dark  there  (T'}.  There  is  certainly  no  such  wisp  or  tail  to  741  as 
BOND  gives. 

1875,  NOVEMBER  17. 

End  i2h.     Mag.  power,  175.     Wt.  =  2. 

685,  708,  741. 

741  has  certainly  no  wisp  or  tail  towards  the  south.  708  and  685  have  none  as 
figured  by  BOND,  but  the  same  appearance  as  previously  described  is  again  seen. 

Pons  Schroeteri  and  ff. 

The  middle  point  of  the  pons  Schroeteri  is  almost  stellar ;  seen  by  indirect  vision 
it  is  (comparatively)  very  bright,  almost  as  much  so  as  the  south  edge  of  6  which,  for 
the  east  two-thirds  of  its  length,  is  noticeably  and  suddenly  brighter  than  the  rest 
of  G. 

Differences  of  R.  A.  from  Ol. 

Coincidence  of  fixed  and  micrometer  wires  64^23  [fixed  wire  on  /91]. 


Object  measured. 

Reading  of 
micrometer. 

AR.A.  in 
revolution. 

AR.A.in" 

Extreme  following  end   of  "  Spitze  " 
of  (T. 

r. 

47-22 

46.90 
47-34 

r. 
[•    +  17.08 

+       169.9 

The  following  end   of   Q   is   in    the 

Mean  47.  15 

+         U1 

same  R.  A.  as  708,  i.  e,, 
Pans  Sthrotteri    .... 

Brightest  part     

The  north  part  of  this  precedes  a  little 
i.  e.,  in  />=  355°  about   

56.04 

\    +     7-97 

+         79-3 

J 

Apex  of  E  almost  exactly  south  of  01 

Mean  56.26 

o  o 

Middle  of  W1     

(Driving  clock  failed).  Refraction  cor- 

3-74 

3/ 
o  o 

rections. 

Order  of  brightness. 

A,  D,  G,  F,  I  =  H,  N,  Q,  E. 

The  south  part  of  F  and  the  middle  of  G  almost  stellar  in  appearance.     N  and  Q 
not  very  well  defined.     These  masses,  in  the  Index-Chart  are  perhaps  not  well  drawn 


OTT. — r  . 


There  are  two  brighter  spots  (elongated)  in  OTT,  as  in  the  Index-Chart.  North  of 
o  it  (the  whole  mass)  it  is  quite  dark,  close  up  to  the  south  edge  of  o. 

Sinus  magnus. — T'  and  V. 

The  darkest  space  in  the  Sinus  magnus  is  bounded  by  the  curved  line  in  Index- 
Chart,  i.  e.,  following  t\\v  pons  Schroeteri  Prolong  OTT  towards  the  west  and  the  space 
south  of  this  line  [prolonged]  and  following  the  pons  is  blacker  than  any  neighboring 
part.  It  is  blacker  than  the  ground  on  which  the  trapezium  stands,  for  example. 
North  of  this  line  (o  n  prolonged)  it  is  brighter,  but  still  very  faint. 

q>  and  6.     (lac us  Lassellii.} 

Between  g>  and  <7  there  is  a  dark  channel;  just  south  of  663  it  ceases  to  be  very 
dark,  and  this  darkest  portion  ends  in  a  curve  convex  towards  the  south.  There  is  a 
dark  channel  connecting  this  with  the  Sinus  or  nearly  connecting  it.  I  think  the  south 
end  of  this  channel  is  wider  than  in  the  Index-Chart. 


r. 


South  third  is  blacker  than  north  two-thirds.     The  bay  in  which  647  is  has  never 
been  seen  so  far. 

Messierian  branch. 

BOND'S  preceding  edge  of  this  branch  is  finely  given.     Its  outline  is  very  hard  and 


I  !6  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

sharp  like  water-color  laid  on  and  allowed  to  dry  at  the  edge.     The  north  edge  of  the' 
rostrum  (/*)  in  BOND  strikes  me  as  too  bright  relatively,  but  I  am  not  sure  that  it  is  so. 

MAIRAN'S  nebulous  star  [No.  734] 

The  shape  in  BOND  is  not  quite  right.  The  brightest  part  follows  and  is  north  (a 
little)  of  the  star  [737].  There  are  two  dark  spaces,  one  on  the  n.  p.  and  one  on  the 
s.  f.  side,  but  not  quite  as  figured;  the  bright  portions  do  not  seem  to  be  exactly  right. 
[Sketch  omitted  of  a  very  faint  star  ^  =  355°  5=24"  (both  estimated)  from  [734]- 
The  very  faint  star  above  given  is  a  very  good  test  for  light. 

1875,  NOVEMBER  24. 

Begin  ioh  iom,  end  nh'30m.  Mag.  power,  175.  Wt.  =4.  647  >  (575)  >  (671) 
or  (676)  >589  >(56;). 

The  line  through  685  and  708  passes  through  the  brightest  part  of  F  (i.  e  ,  X  [?]) 
and  is  parallel  to  the  black  channel  between  F  and  (G  and  H).  [Such  remarks  as  this 
are  always  founded  on  an  observation  of  a  micrometer  wire  laid  through  the  stars.] 


OTT     T  —     — 


TT  is  brighter  than  o  /  o  is  brighter  than  the  middle  of  o  TT.  North  of  o  ^  it  is  black  ; 
preceding  it  is  black  ;  south  also  black  [a  narrow  channel].  After  the  channel  south  of 
OTT  is  crossed,  the  Sinus  is  filled  with  nebulosity  up  to  its  south  border.  Just  following 
pons  Schroeteri  it  is  very  black.  Half  way  from  o  to  south  border  of  Sinus  there  is  a 
nucleus,  very  faint  and  almost  stellar,  more  nearly  stellar  than  the  nucleus  in  pons 
Schroeteri. 

G. 

Folloiving  the  "  Spitee"  in  same  parallel,  there  is  a  star  whose  distance  from 
point  of  Spitze  is  equal,  approximately,  to  the  distance  of  the  latter  from  669. 

Order  of  brightness. 
A,  D,  G=H,  F,  N,  Q,  I,  E,  J. 

Sinus  Lamontii. 

Although  the  Sinus  Lamontii  is  plain,  yet  the  Harvard  College  Observatory  drawing 
of  1874  exaggerates  the  effect  as  seen  now. 

Channel  between  cp  and  G.     (lacus  Lassellii.) 

This  black  channel  certainly  goes  from  652,  663  through  to  the  Sinus  ma  gnus. 
It  is  rounded  south  of  663,  and  black  ;  then  faint  and  wider  than  the  Index-Chart  has 
it  up  to  the  Sinus.  It  is  about  as  bright  as  north  half  of  pons  Schroeteri. 

567. 

567  is  in  a  black  space  half  way  between  B  and  i.  575  and  589  in  two  bright 
streaks,  as  in  sketch  of  1875,  October  27. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


South  shore  of  rj  is  about  right  in  Index-Chart. 

Dark  channel  between  E  and  F. 

p  (estimated)  140°.  Position-circle:  293°.5,  290.0.  Mean,2qi°.'j.  .'.p 
This  is  the  angle  of  position  of  the  dark  channel  between  E  and  F;  its  direction 
passes  through  the  star  589.  Hence  the  Index-Chart  is  wrong  in  this  point.  It  passes 
in  direction  p  —  140°,  and  then  turns  sharp  off  towards  63  1.  The  [vertex  of  the]  angle 
is  sharp,  not  rounded  as  in  Index-Chart. 

671,  676!  [possibly  671-686!  ?]  prolonged  intersect  the  from  in  a  bright  nodule. 

The  pastel  drawing  of  the  Naval  Observatory  has  the  dark  channels  better  than 
the  Harvard  College  Observatory  drawing  of  1874  [both  these  are  by  M.  TROUVELOT]. 
The  channels  are  pretty  wide  and  pretty  well  defined  at  edges. 

The  dark  channel  following  D  does  connect  with  Sinus  maynus. 


r. 


The  preceding  and  south  edges  of  T  are  bordered  by  a  very  black  stripe ;  then  the 
south  third  is  black,  its  north  two-thirds  full  of  nebulosity.  In  the  Harvard  College 
Observatory  print  of  1874  the  reverse  is  the  case ;  i.  e.,  it  is  darker  at  the  north  end. 

635,  7o8,  741. 

The  same  remarks  as  formerly  made  apply  to  these  stars.  It  is  dark  between 
them,  and  brighter  just  south  of  them,  but  my  previous  sketches  [omitted]  are  right, 
or  nearly  so.  MAIRAN'S  nebulous  star  [No.  734].  This  nebulous  star  has  certainly 
altered  since  BOND'S  time.  Between  the  principal  star  [734]  and  that  one  at  the 
point  of  the  comma  [785]  there  is  a  broad  dark  streak  extending  quite  across  the 
nebulosity  and  dividing  it  into  two  parts.  Its  direction  is  s.  f.  to  n.  p. 

P> 

Following  the  "  Spitze"  there  is  a  curious  repetition  of  the  prow-like  shape  of  the 
Spitze  itself;  it  is  much  fainter,  and  is  close  to  fft  so  that  it  looks  like  the  shadow  of 
it,  a  little  distorted. 

1876,  JANUARY  3. 

Mag.  power,  1 75  ;  end  i  ih  45"     Wt  =  4-     Seeing  very  good. 

Order  of  magnitude. 

i.  (575)-  2  (671)  or  (676)  [probably  (671)].  3.  622  or  625.  647  ?  is  brighter 
than  any  of  these.  It  is  in  a  dark  space  surrounded  by  bright  nebulosity  even  towards 
the  east,  and  this  [nebulosity]  is  suddenly  much  brighter  towards  the  north. 


r. 


The  north  half  is  filled  with  faint  nebulosity;  the  south  half  is  empty.     Halfway 
between  the  foUowing  edge  of  D  and  the  preceding  edge  of  pons  Schm,  t,  rl  there  is  cer- 


I  1 8  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

tairily  a  faint  bright  bridge  of  light,  as  sketched  [in  by  me]  on  the  Index-Chart,  similar 
to  SCHROETER'S  second  bridge.  [See  Beitrage  zuden  neuesten  astronomischen  Entdeck- 
ungen.~\ 

[Its  base  is  connected  on  the  north  to  v],  and  it  extends  towards  the  south  as  far 
as  the  parallel  of  the  bright  nucleus  of  the  pons  Schroeteri. 

Pons  Schroeteri  (g0). 

This  nucleus  is  seen  stellar  beyond  a  doubt ;  not  the  whole  nucleus,  but  a  point 
inside  the  central  condensation. 

* 

T. 

The  preceding  side  of  r  is  the  blacker. 
[There  is  a  black  stripe  bordering  D  and  T.] 

Order  of  blackness  (not  brightness). 

i.  Space  following  pons  Schroeteri  and  preceding  £  [r'].     2.  Space  between  6  and 
o  TT  [r"].     3.  Space  in  which  trapezium  is  situated  [V].     4.  South  half  of  T.     5.   £. 
N.  B.  This  makes  south  half  of  T  brighter  than  usual,  but  it  is  right. 

fji  and  6. 

Rostrum  —  >u  is  about  as  bright  as  6.  [I  suppose  this  to  mean,  as  bright  as  the 
main  body  of  6  and  not  as  bright  as  the  brighter  southern  edge  of  it.  1877.] 

Order  of  brightness. 
i.  A.     2.  D.     3.  G,  H.    4.  N,  Q,  F,  I.     5.  E.     (654)  not  seen,  although  looked  for. 

1876,  JANUARY  4. 
ioh  20m.     Mag.  power,  175 

Order  of  magnitude. 

i.  (671)  or  (676).     2.  (575)  almost  zz  671.     3.  622?.     4.  589.     5.  567. 
612  =  618  zz:  (676)  ;   581  and  R.  56  both  seen;  also  636,  but  not  (654). 

Order  of  brightness. 
i.  A.     2.  D.     3.  G,  H,  N.     4.  F.     5.  I.     6.  E  :  and  I  =  Q  nearly. 

Rather  hazy,  and  work  unsatisfactory.  At  the  same  time  the  small  stars  are  seen 
very  well  indeed.  No  signs  of  LASSELL'S  b  [between  685  and  708].  709  seems 
rather  fainter  than  usual ;  I  noticed  this  also  last  night,  January  3. 

1876,  JANUARY  10. 
nh  3om-i2h.  Mag.  power,  175.     Wt.  =  i. 

Order  of  brightness. 

i.  A.  2.  D.  3.  G,  H,  N,  Q.  4.  F.  5.  I,  E.  Not  very  good.  Strong  moon- 
light, and  details  faint.  Sinus  Lamontii  appears  very  strongly  marked  to-night,  much 
as  in  Harvard  College  Observatory  drawing  of  1874.  This  is  simply  on  account  of  the 
moonlight,  and  not  that  it  is  really  any  stronger.  [This  remark  is  based  on  a  great 
many  observations  in  the  years  i873-'74-'75-'76.]  The  night  is  so  bad  that  [much] 
further  work  is  impossible. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  UQ 

A  line  in  the  Huyghenian  region  parallel  to  the  frms  and  passing  through  602 
would  pass  through  a  region  which  is  fainter  than  those  on  either  side  of  it;  that  is, 
there  is  a  fainter  bar  running  through  the  whole  Huyghenian  region. 

T  —  T'  —  y. 

Even  to-night  I  can  see  that  the  east  side  of  T  is  not  so  black  as'the  west,  and  that 
the  north  is  not  so  black  as  the  south,  r'  is  to-night  of  about  the  same  blackness  as  r 
and  as  7,  but  there  is  very  little  weight  to  be  given  to  work  done  under  such  condi- 
tions. Nearly  full  moon,  and  seeing  extremely  bad. 

1876,  JANUARY  30. 

ioh.^  Mag.  power,  400 :  seeing  not  good.  Wt.  =  2.  (The  original  paper  on  which 
observations  were  recorded  has  been  mislaid,  and  the  following  is  from  memory  [and 
of  course  is  only  a  part  of  work  done],  but  I  am  certain  of  everything  recorded.) 

Pons  Schroeteri. 
Center  distinctly  concentrated ;  almost  stellar. 


on. 


0  and  TT  like  nuclei,  verging  towards  a  stellar  appearance  but  not  so  much  as  cen- 
ter of  pom. 

T"  and  T. 

Quite  black  below  [north  of]  OTT  and  on  preceding  side  of  T.  A  thin  black  streak 
edges  all  the  west  and  north  sides  of  T,  but  the  north  half  of  r  is  decidedly  nebulous, 
while  the  south  half  is  almost  jet  black. 

W1  and  V. 

W1  =.  lacus  Secchii  seemed  blacker  than  V  —  space  around  trapezium. 

ff. 

The  south  edge  of  G  has  a  quite  sharp  bright  termination ;  i.  e.,  comparatively 
much  brighter  than  v,  for  example. 

The  black  space  around  647  is  not  entirely  black,  and  perhaps  it  is  too  well 
marked  on  Naval  Observatory  pastel  drawing  by  TROUVELOT. 

V. 

1  should  say,  also,  that  V  was  too  black  in  that  drawing.     Two  sequences  of 
brightness  of  masses  in  Hnycjhenian  region  recorded  which  I  cannot  exactly  remember 
and  therefore  do  not  record,  but  both  agreed  in  making  E  quite  faint. 

1876,  FEBRUARY. 
Begin  7h  3Om,  end  8h  30™.  Power,  400.     Wt.  =  4  at  first,  then  =  i. 


I2O  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Order  of  brightness. 
i.  A.     2.  D.     3.  0,  H,  F.     4.  N.     5.  I  Q  or  Q  I.     6.  E. 

Order  of  blackness. 

i.  T'  —  y.     2.  T  (south  half  only).     3.  V.     4.  North  half  r.     Again:   i.  T'.     2.  T' 
3.  7.     4.  /SbtttfA  half  r.     5.  V.     6.  jVbr$  half  r. 

Differences  of  declination. 


Object  observed. 

Micrometer 
reading. 

A(5  in  revo- 
lutions. 

A<5  in  seconds. 

71  .54 

—       14.  3<? 

—    142    6    ) 

South  point  of  F       

60.40 

•TB  >JJ 
12.28 

•  •§••**   i 
(-   '42.  7)  I" 

—     122  .  2    ) 

7  .  tfy 
67.51 

—       10.30 

(-     122.3)} 

—   102  <;   ) 

v  .  J^* 

*  *-**•  o    r 
(-    102.6)  ) 

This  is  about  the  same  declination  as 
north  point  of  E. 

•            • 

'    • 

(—    IO2.6) 

Through  the  center  and  brightest 
part  of  G. 

64.21 

—     \  7.00 

(-  69.6) 

North  end  of  Sinus  Gentilii  *  . 

63.9 

-         6.7- 

(-     66.7) 

South  end  of  A  near  star  622  . 

60.41 

—         3-20 

(-     31-8) 

North  end  of  L  on  same  parallel  as 
671  and  the  east  point  of  Q. 

6}  Orionis  

59-78 

57.21 

-         2.57 
o.oo 

(-     25.6) 
(         o  o) 

Middle  of  break  \npons  Schroeteri     . 

j  /  • 
55-0 

+            2.21 

\                **•  v/ 

(+      22.0) 

Brightest  part  of  fans  Schroeteri    . 

53-0 

+           4-21 

(+      41.9) 

All  north  of  this  in  r  is  filled  with 
nebulosity,  except  of  course  the 
black  channel  on  its  west  edge  ; 
nearly  all  T  south  of  this  is  pretty 
black,  though  not  so  black  as  r1. 

52.48 

+           4-73 

(  +     47-1) 

About  through  the  center  line  of  west 
half  of  OTT:  this  parallel  passes 
through  the  break  in  pons  Schroeteri 
north  of  its  nucleus. 

51.18 

+           6.03 

(+     60.0) 

South  side  of  a  (approximately)  . 

48.8 

+           8.4- 

+     83.6   ) 
(+     83.7)1 

South  end  of  E  ;  seeing  growing 
worse. 

71.78 

-         14-57 

-   145-0  ) 

(-  MS.  oi 

South  end  of  E  ;  seeing  growing 
worse. 

71.69 

14.48 

-  144.1  i 

(-  144.  2)  y 

The  J6's  corrected  for  refraction  are  inclosed  in  brackets. 

Now  put  on  mag.  power,  175,  all  the  measures  having  been  made  with  400. 

No  tails  or  wisps  to  708-741  to-night. 

G.  P.  BOND'S  Eegio  subnebulosa  is  right.  The  rostrum  [/*]  in  Naval  Observatory 
drawing  of  1875  i§  too  narrow.  Made  sketch-map  of  stars  around  and  in  MAIRAN'S 
nebula  [omitted]. 

*  [Drawing  omitted.]  The  Sinus  Gentilii  is  connected  by  a  black  channel  running  to  the  N.  E.,  with  space  near 
trapezium.  V  is  almost  perfectly  black  *  *  [in  sketch]  is  a  little  brighter  [***...  is  a  narrow 

space  bordering  V  on  its  preceding  side],  and  on  its  west  edge  is  a  little  ridge  [brighter],  and  west  of  this  is  the  black 
channel,  connecting  with  that  just  mentioned  [i.  e.,  that  one  running  to  the  N.  E.]. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  121 

1876,  MARCH  6. 
8h-9h.  Mag.  power,  175.     Wt.  =  2.     Parallel,  i5i°.o. 

Occiput. 

Angle  of  position  of  preceding  edges  of  E  and  I. 

Position-circle:   102°,  101°,  97°.6,  96°4;  mean,  99^3  .  •  .p  —  142°.;. 

Dark  channel  between  F  (on  the  south)  and  G  and  H  (on  the  north). 

Its  prolongation  passes  nearly  through  685  and  708,  whether  exactly  or  not  the 
night  is  not  good  enough  to  determine. 

Spiral  formation  of  the  nebula  well  seen 

OTTO  v.  STRUVE'S  node  of  nebulosity,  by  O.  2.  126  (G.  P.  B.  793),  is  not  seen; 
[referring  to  an  observation  communicated  by  letter].  (575)  >  589  >  (567).  The 
last  star  is  quite  faint. 

622. 

Angle  of  position  of  622  from  0'. 
Position  circle  :  42°.o  .  * .  p  —  i99°.o. 

The  line  of  622  and  0'  passes  through  (or  nearly  so)  a  bright  star  south  of  Huy- 
ghenian  region  6'  [570  G  P.  B.]. 

Dark  channel  between  I  (on  west  side)  and  X  and  G  (on  east  side). 

This  is  a  straight  portion  of  some  length  whose  direction  goes  through  G.  P.  B. 
5  70  (same  star  as  noted  just  above). 

Position-circle:   213°. 7,  2i5°.7,  216°. 8;  mean,  2i5°.4  .  • . p  —  25°. 6. 

c. 

The  direction  of  the  spiral  c  (towards  523)  is  about  right  on  Index-Chart.  The 
seeing  is  not  good  on  account  of  haze,  and  the  weight  of  the  measures  is  small. 

1876,  MARCH  14. 

S  PRUVE'S  new  nucleus  near  793. 

J6  micrometer  wire  on  793     -  89.91 

wire  B  on  new  nucleus    -----        93.90 


Again:  9ai3 

93-90 


-    3.77  =  3 


J6  mean  38".S 

This  nucleus  precedes  793  =  (O.2. 1 26),  and  is  quite  faint  through  the  light  clouds 
which  cover  the  sky,  still  it  can  be  steadily  seen  in  a  dark  field,  but  not  quite  steadily 
APP.  V 16 


122 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


on  the  bright  wire;  [the  p  (estimated)  of  this  object  from  793  was  about  190°  to  200°, 
and  it  is  probably  not  0.  JS.'s  nucleus]. 

[Sketch  of  793  and  vicinity  omitted.] 

The  small  space  [just  following  793  and  about  20"  to  25"  in  diameter]  seems  to 
glisten  with  bright  small  points,  a  little  like  the  Huyghenian  region  just  south  of  trape- 
[Just  north  of  793  and  immediately  preceding  the  Messierian  branch]  there  is  a 


zium. 


dark  channel  which  separates  the  branch  from  the  diffused  nebulosity  of  the  Eegio  sub- 
nebulosa.     This  channel  is  like  BOND'S  dark  channels  in  nebula  of  Andromeda. 

Order  of  brightness. 

i.  A.  2.  D.  3.  F,  G,  H.  4.  N.  5.  I.  6.  Q,  E,  not  a  very  satisfactory  order 
in  spite  of  good  seeing.  [Probably  just  on  account  of  good  seeing,  so  many  details 
confuse  a  general  judgment]  i.  A.  2.  D.  3.  F.  4.  G,  I.  5.  H,  E.  6.  N,  Q. 

This  is  better,  but  the  mag.  power  400  shows  too  many  details  to  assign  this 
order  satisfactorily. 

V.  -  r'. 

V.  is  by  no  means  as  black  as  the  Sinus  Gentilii,  but  comparable  with  [in  black- 
ness] and  almost  equal  to  the  north  half  of  r' '. 

Differences  of  right  ascension. 

575  is  just  exactly  north  of  573  [according  to  BOND  575  follows  573,  3"],  and  the 
line  joining  them  skirts  along  the  preceding  shore  of  Smus  Gentilii,  and  is  the  best 
tangent  to  this  shore.  On  re-examination  I  find  573  preceding  575  by  not  more  than 
o//-5  [3"  according  to  BOND].  The  line  of  shore  between  (/?  and  K)  and  y  is  curved, 
though  not  quite  so  strongly  as  it  is  drawn  in  Naval  Observatory  drawing,  1875  ;  y, 
in  that  drawing,  needs  to  be  moved  bodily  towards  the  east  to  conform  to  the  line 

573-575- 

setting  241°.!,  wire  A  on  685,  and  at  64''.  1 8. 


Object  measured. 

• 

Micrometer 
reading. 

A  a  from  685 
revolutions. 

A«  from  685 
seconds. 

A  a  from  01 
seconds. 

82.62 

r. 
18.44 

ii 

183.4 

i/ 
-         86.5 

Preceding  side  of  \Vi     ..... 

70.68 

—       15.50 

154.2 

—         57.3 

This  declination-circle  bisects  the 
rounded  apex  of  the  mass  I,  and 
limits  W1  on  its  following  side  ; 
(W1  narrows  here  on  the  following 
side  to  something  like  a  canal.) 

76.85 

—       12.67 

—       126.0 

—         29.1 

Bisects  the  rounded  apex  of  E  (not 
a  very  good  observation)  and  passes 
through  the  northern  star  of  t  h  e 
trapezium. 

74-45 

—       10.27 

—         102.2 

5-3 

Tangent  to  the  preceding  sides  of  F 
and  G  and  passes  between  61  Orie- 
nts and  the  6th  star. 

73-  ^ 

—         9.62 

—        95-7 

+              1.2 

Tangent  to  brightest  and  following 
side  of  D  ;  tangent  to  following  side 
of  G  and  bisects  F  very  nearly. 

71.02 

6.84 

-         68.1 

+           28.8 

Nucleus  of  pans  Schroeter.i, 

66.24 

—         2.06 

2.5 

+            76.4 

N.  B.—  Aa  of  685  =  +  96".g. 

MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  123 

End  8h  3om.     Power  400,  used  throughout.     Sky  hazy,  and  images  quite  steady. 

793- 

Looked  at  793  again  ;  saw  my  star,  but  nothing  else  steadily,  although  the  space 
/(Mowing  is  certainly  full  of  bright  points. 

1876,  MARCH  22. 
7h  40.m-8h,     Mag.  power,  400.     Wt.  =  3. 

793- 

I  see  s.p.  793,  5  =  40"  the  faint  star  previously  measured  (March  14);  this  is,  of 
course,  preceding  the  bright  line  of  [west]  nebulosity  of  the  Messierian  branch. 

Inside  of  the  branch  [and  near  793]  I  am  not  sure  of  any  point.  There  may  be 
one  20//-25//  off,  a  little  following,  but  the  night  is  not  good  enough  to  decide.  Just 
preceding  the  Messierian  branch  from  793  north  to  784  there  is  a  black  streak  of  varying 
width  (not  more  than  i')  which  extends  from  793  towards  the  'north.  [Drawing 
omitted.] 

Order  of  brightness. 
i.  A.     2.  D.     3.  F.     4.  G,  I,  H.     5.  N,  Q,  E.     (Not  a  very  careful  observation.) 

\ 

1876,  APRIL  i. 

8b.     Very  poor  seeing.     Mag.  power,  1 75. 

709  is  a  little  harder  to  see  than  the  5th  star.     [Query,  6th  star?] 

709  =  671  or  647  about.     It  requires  attention  to  see  it. 


T. 


North  half  much  brighter  then  south  half. 
Seeing  too  poor  to  go  on. 

1876,  NOVEMBER  5. 

•s 

Mag.  power,  400.     End  i4h.     Wt.  ~  3. 

675  visible,  arid  well  involved  in  nebulosity. 

Figure  [omitted]  shows  two  nuclei  in  N  (denoted  by  a  and  b  in  this  night's 
work),  and  the  nuclei  686,  688  with  stars  671  and  676.  There  is  a  dark  space  between 
671  and  676.  (671)  >(6;6). 

South  of  the  line  671-676  it  is  black,  and  676  seems  to  be  on  the  preceding  edge 

of  P  or  0. 

The  dark  channel  between  0  and  P  not  well  seen  (night  not  good). 

Q. 

The  nort-h  side  of  Q  quite  bright  and  sharp. 


T. 


The  second  bridge  of  SCHROETER  is  seen  much  as  I  have  drawn  it  before. 


124  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA   OF  ORION. 

a.  &.    686,  688. 

i3h  5om-  The  line  joining  676  and  685  passes  a  little  east  of  the  points  a  and  I. 
These  look  nebulous,  as  do  686  and  688  of  the  figure  and  not  like  stars,  a  is  on 
the  following  edge  (the  exact  edge)  of  N  and  688  is  on  the  north  edge  of  Q. 

636  visible  but  not  654. 

The  night  is  bad  and  work  not  satisfactory. 

1876,  NOVEMBER  n. 
1 2h.     Bad  seeing. 

Order  of  brightness. 

H,  G,  F  (about  equal),  I,  E. 

Dr.  C.  S.  HASTINGS  sees  the  second  bridge  of  SCHROETER. 

1876,  NOVEMBER  22. 

Begin  ioh  45™,  end  i  ih  15™.     Mag.  power,  175.     Wt.  zr  2. 

Frons. 

Position-circle:     59°4,  59°. 2,  59°. 8,  6o°.o;   mean,  59°  .6.     Parallel  —  1 08°. 9 
p  =  49°.3  (4). 

Occiput. 

Position-circle:   i52°.9,  i52°.9,  i52°4,  i5i°.2;  mean,  i52°-3;  P  —  i36°-6  (4)- 
Best  tangent  to  south  shore  of  Sinus  magnus.     (To  the  general  direction  of  the 
shore,  cutting  off  a  little  of  the   south  end  of  pons  Schroeteri.) 

Position-circle:   i82°.6,  i8o°.8,  i86°.o,   i83°.9;    mean,  i83°.3;  p  —  iO5°.6  (4). 

1876,  NOVEMBER  27. 

Begin  ioh  3Om,  end  nb  om.     Mag.  power,  i75=Jb     Wt.  —  i. 

Moonlight  and  flying  clouds,  which  finally  prevent  work.     Preceding  edges  of 
J  and  B  (through  575). 

Position-circle:    103°. 8.     Parallel,  io8°.9  ;  p  —  5°. i  (i). 

1876,  DECEMBER  5. 


Begin  i2h  om,  end  i2h  45™.    Mag.  powers,  400  and  175.    Wt.  •=.  i.     Parallel  •=. 


is0-*. 


Differences  of  decimation  with  9'  and  brightest  part  of  G. 


7°r-55>  7°r-78,  7or.6i,  70^87,  70^69;-  mean,  70^70 

Zero,  64r.i2 


Poor  measures.  J6—    6r.58  —  65/7.5  (5). 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  125 

Dark  channel  between  I  and  E. 

Its  prolongation  precedes  671. 

Position-circle:    6o°.9,  62°.!,  64°.o,  63°.2 ;  mean,  6 2°. 6  (4).    p  —  42°.6  (quite 
uncertain).     First  three  measures  with  eye-piece  400;  last  with  175. 
647  >  (671),  (575)- 

68 1  >  (676)  >  651  ;  not  much  difference  in  these ;   709  =.  663  about. 
Very  cold  and  seeing  bad. 

1876,  DECEMBER  13. 
Begin  i2h  40™,  end  13*  om.     Mag.  power,  175+- 

Order  of  brightness. 

i.  A.     2.  D.     3.  G,  H.  F.     4.  N,  I,  E. 

Mr.  H.  S.  PRITCHETT  puts  D  and  A  about  equal,  but  thinks  D  a  little  the  brighter. 
The  space  between  685  and   708  is  blacker  than  that  between  708  and  741. 
Clouds. 

1876,  DECEMBER  19. 
Begin  13*  15"  end  i3h  35™.     Wt.  =  3. 

Order  of  brightness. 
(Mr.  PRITCHETT.)     i.  DnR.     2.  A.     3.  I.     4.  H  and  F.     5.  B. 

SCHROETER'S  second  bridge. 
It  extends  to  the  south  as  far  as  the  parallel  of  647. 

T:  T'. 

The  south  half  very  black;  T'  blacker  than  north  half  of  T. 

Order  of  brightness. 

i.  A.    2.  D.  3.  F.     4.  Q.     5.  N.     6.  G.     7.  H.     8.  I.     9.  E. 
B  =  F  =  W ;  Y  =  0  (doubtful). 

Order  of  blackness. 
i.  T'.     2.  T".     3.  South  half  of  r.     4    W.     5.  Sinus  Gcntilli 

Sinus  Gentilii  brighter  than  Wl. 
685-708  in  the  prolongation  of  the  dark  channel  between  (F  and  G)  and  H. 

7- 

As  in  Index-Chart. 

Pons  Schroeteri. . 

Its  direction  passes  through  685  nearly. 
Position-circle:  325°-7,  323°-5,  323°-O;  mean,  324°. i. 
p—iJiQ.2.     (3). 


126  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Preceding  edges  of  J.  and  B. 
Position-circle:  301°. 5,  303°. 7,  3O4°.o;  wean,  303 °.i.     (3). 

p—l2°.2.       (3). 

[Drawing  made,  omitted.] 

This  drawing  shows  the  portion  of  the  Huyghenian  region  following  the  meridian 
of  & ',  as  it  appears  in  a  general  view.  It  was  made  particularly  to  show  a  darker 
band  which  rests  on  T,  as  a  base,  and  extends  towards  the  south,  ending  on  the  frons 
between  H  and  N;.  This  part  is  about  as  bright  (in  general)  as  the  dark  channels. 
It  is  apparent  at  first  glance  in  a  general  view,  and  the  drawing  gives  its  general  shape. 
It  looks  like  a  continuation  of  r.  In  this  drawing  the  second  bridge  of  SCHROETER 
extends  south  to  the  parallel  of  647;  651  is  precisely  on  the  edge  of  T,  \i.  e.,  da  of 
tangent  to  following  edge  of  D  —  2 8' '.8]  ;  g0  is  shown  as  a  central  nucleus,  surrounded 
by  an  annulus,  etc. 

T. 

The  south  third  of  T  is  black,  but  I  seem  to  be  aware  of  one  or  two  bright  stellar 
points  in  it,  which  I  cannot  fix,  but  which  I  believe  to  be  real. 

SCHROETER'S  second  bridge. 
Its  position  angle  is  a  little  greater  than  that  of  pons  Schroeteri. 

Lacus  Lassellii. 

It  is  connected  with  Sinus  magnus. 

Y. 

Quite  bright  and  equal  to  6  near  k. 

2. 
Contains  a  bright  star  [570]. 

B. 

Extends  no  farther  than  575  as  a  bright  mass,  certainly  not  as  much  farther  as  is 
given  by  Lord  ROSSE  (1867). 

A. 

The  convolutions  in  A  on  its  following  edge  give  the  effect  of  LASSELL'S  drawing 
of  1862  [unpublished,  but  most  courteously  communicated  to  me  in  a  full  size  pencil- 
copy  by  Miss  CAROLINE  LASSELL],  but  some  of  the  details  are  different  now. 

1876,  DECEMBER  31. 

Begin  ioh  7™,  end  nb.  Mag.  power,  150  (Ax).  Wt.  —  2  at  beginning.  (Moon- 
light.) Sky  very  clear,  and  seeing  improving  a  little  toward  the  end. 

709  <  657,  657  —  652  about-.  647  and  651  as  in  Index-Chart.  (671)  >  676; 
575  —  589,  although  575  is  first  caught  by  the  eye  on  account  of  its  situation,  being 
more  free  of  nebulosity.  Both  575  and  589  less  bright  than  647  and  671,  although 
647,  671,  and  575  are  not  very  unequal. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  j  27 

B,  A. 

B  runs  exactly  through  575,  and  if  it  extends  beyond  (south)  of  575  as  a  distinct 
mass  it  is  considerably  fainter.  589  appears  at  the  south  end  of  a  bright  part  of  A,  and 
between  575  and  589  is  an  oval  dark  gulf.  [Drawing  of  convolutions  in  A  omitted.] 
The  branch  (B)  leading  to  575  is  brighter  than  that  leading  to  589. 

V  and  A. 

Dark  channel  between  V  and  A,  much  as  in  Index-Chart.  It  leads  around  toward 
the  west  in  a  very  regular  curve,  connecting  with  the  dark  space  north  of  c.  V  con- 
tains 612  or  618  (the  brighter  of  these  two)  [618]. 


Wi  connects  with  the  dark  channel  north  of  2,  which  runs  towards  524. 

W4. 

W4  is  larger  and  more  toward  the  west  than  in  Index-Chart.  Wj  blacker  than  W4f 
but  not  much,  and  both  much  blacker  than  W3,  which  to-night  is  not  dearly  outlined. 

Telescopic  meteor  crossed  lower  half  of  the  field  of  view  (25'  in  diameter)  from 
s.  p.  to  n.  f.j  position-angle  about  50°  very  rapidly,  lasting  about  o8.  i  ;  as  bright  as 
724.  Channel  just  north  [south  ?~\  of  I  is  about  parallel  to  frons. 

685-708  prolonged  is  in  direction  of  channel  between  (F  and  G)  and  H. 

Dark  channel  preceding  M  has  a  direction  from  685  to  a  point  about  half  way  from 
618  to  647.  To:night  it  seems  to  extend  and  join  with  V. 

686  seen  well.     It  is  probably  a  cluster  of  very  small  stars  or  a  nebulous  nucleus. 

The  north  half  of  the  second  bridge  of  SCHROETER  seen.  T  is  dark,  in  the  same 
Jd  as#0. 

1877,  JANUARY  2. 

Begin  9h  4Om,  end  iob  2Om.  Temp.  =  2°.  5  F.     Mag.  power,  175  ±.     Wt.  n  2. 

Order  of  brightness. 

i.  A.     2.  D,  G,  F.     3.  H,  I,  N.     4.  E.     E  >  M  >  o  >  /*. 

B,  about  half  way  from  575  to  its  north  end,  is  about  as  bright  as  the  general  mass 
of  I  ;  but  this  comparison  is  very  hard  to  make.  J  very  faint. 

J  <  space  just  south  of  W3  and  about  W4,  and  J  <  E,  but  J  >  G. 

Order  of  blackness. 
T'  —  T"  •   r"  blacker  than  W1?  which  is  blacker  than  the  south  half  of  T. 

SCHROETER'S  second  bridge. 

It  is  seen  more  like  ROSSE'S  figure  than  before.  Only  the  south*  two-thirds  seen. 
558  >  709  >  524- 

*  For  south  read  north  probably. 


128  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

C. 

c  is  about  as  bright  as  B  (roughly  speaking)  a  little  preceding  575,  and  it  joins  a 
little  more  smoothly  into  the  preceding  edge  of  A. 


It  is  uniformly  filled  with  v.  F.  nebulosity. 

(575)  =  671  =647,  nearly. 

6  and  y  as  in"  Index-Chart. 

The  nuclei  of  F  and  Gr  were  stellar  in  appearance  ;  more  so  than  usual,  and  in  a 
less  degree  those  of  I  and  H.  E  is  always  nebulous  and  uniform  in  brilliancy,  if  we 
except  the  very  small  surfaces  which  give  what  HERSCHEL  calls  the  "appearance  of 
stippling"  to  the  whole  nebula,  and  which  are  not  to  be  fixed  in  position. 

Y. 

Y  is  not  a  marked  feature  of  E,  as  in  LASSELL  (1862),  but  requires  a  little  atten- 
tion to  see  it. 

Y- 

y  is  pretty  uniformly  black,  but,  of  course,  not  so  black  as  r',  etc.,  but  is  uniform. 

77. 

570  (in  <?)  has  blackness  (;;)  just  south  of  it,  arid  this  precedes  it.  The  blackness 
north  of  it  (between  e  and  <?)  precedes  it  but  little. 

The  general  effect  of  the  spirals  north  of  regio  Huygheniana  is  much  as  in  LASSELL, 
1862. 

P- 

Just  north  of  the  Spitee  and  following  ff  it  is  pretty  black  to-night. 
654  and  675  have  not  been  seen  (though  675  has  hardly  been  specially  looked 
for)  since  November  i. 

M,  etc. 

The  channel  just  following  M  seemed  in  the  bad  seeing  to  be  more  nearly  north 
and  south  than  on  December  3  1  ,  and  to  run  from  its  south  point  in  the  frons  (correctly 
laid  down  on  Index-Chart)  south  through  671  and  676  (which  are  on  a  black  ground) 
to  the  Sinus  magnus.  This  appearance  would  be  produced  if  the  following  end  of  M 
were  faint  from  bad  seeing. 

E. 

The  preceding  part  of  R  quite  faint  to-night.  Among  other  experiments  I  tried 
reflecting  the  image  of  the  nebula  after  it  had  passed  through  the  eye-piece  through  a 
90°  prism.  This  had  the  general  effect  of  bringing  the  point  Q  further  into  //,  and  of 
making  the  appearance  of  the  frons  more  like  HERSCHEL'S  drawing  of  1837. 

Other  experiments  seemed  to  indicate  that  in  a  weak  telescope  the  opening  of  the 
jaws  would  be  determined  by  the  present  south  shore  of  Sinus  magnus  and  the  line 
618-669. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  :  29 

1877,  JANUARY  5. 

Begin  9h  om,  end  9h  3om.     Mag.  power,  i  75  ±.     Wt.  =  i.     Parallel  -  i8°.o. 

Very  poor  seeing ;  unsteady  and  not  transparent.  The  Hemicylium  Liaponovii  is 
plainly  to  be  traced  under  these  conditions.  The  north  side  of  I  appears  to  connect 
with  the  south  end  of  A,  and  this  brighter  part  curves  round  toward  the  west  as  far  as 
589  about  (this  star  not  seen),  then  returning  on  itself  the  folloiving  side  of  A  appears 
to  connect  with  c.  c  plainly  projects  beyond  the  main  Huyghenian  region.  B  also  is 
plainly  bright,  so  that,  beside  the  Sinus  Lamontii,  there  is  a  similar  gulf  between  B  and 
the  following  side  of  A. 

Occiput. 

Position-circle:  i52°.9,  i5i°-8,  152° 7i  mean,  i52°.5.  p  =  135°.$  (3);  this  cuts 
offY. 

Distance  of  occiput  from  & . 

74r-9i;  74r-74:;  74r-85  I  mean,  7^.83 

zero, .  64  . 1 2 

s  =  ior.7i  —  1 06' '.5 
Pons  Schroeteri. 

Position-circle:   I23°.o::;    I2i°<4::;  mean,  I22°.2.     p  —  165°. 8  : :  (2). 
This  measure  gives  really  the  p  of  a  line  joining  669  and  gQ. 
I  chose  this  very  poor  night  for  measuring  the  distance  from  occiput  to  &,  as  the 
nebula  looked  more  like  LAMONT'S  drawing  than  I  have  ever  seen  it.     The  distance  as 
I  measured  it  is  io6".5.     LAMONT  measured  it  twice,  and  obtained  97".8  and  96". 61. 
HOLDEN-LAMONT  =  -\-  8". 7  and  -f-  9/x.9.    LIAPONOFF  obtained  96".4.    HOLDEN-LIAPON- 

OFFn  +   IO".I. 

Comparison  of  La/months  drawing  with  the  nebula. 

The  bay  of  LAMONT  I  see  as  he  did.  B  is  not  laid  down  by  him ;  it  is  plainly 
seen  to-night,  and  its  absence  from  his  drawing  accounts  for  all  the  difference  in  the 
region  between  589  and  575.  His  "  E  "  is  far  brighter  than  it  is  to-day,  and  his  "  I  " 
is  far  more  conspicuous  than  now.  His  "F"  is  nearly  round,  whereas  it  is  now 
triangular.  The  channel  north  of  it  is  to-night  in  the  prolongation  of  685  and  708, 
but  it  is  not  so  in  his  drawing. 

His  H,  if  laid  down  at  all,  is  fainter  than  to-night.  The  north  side  of  A  near  W 
is  to-day  much  brighter  than  in  LAMONT'S  figure.  Locus  Lassellii  is  not  figured  in  his 
drawing.  To-night  it  was  very  prominent.  The  same  remark  applies  to  a  as  a  whole. 
The  extension  of  Q  into  ju  was  remarked  to-night  to  be  like  LAMONT'S  figure. 

1877,  JANUARY  10. 

Begin  9h  i5m,  end  iofl  is"1.     Mag.  power,  175  ±.     Wt  =  2. 
The  night  very  transparent  although  unsteady.     ROSSE'S  drawing  taken  to  the 
telescope  and  compared. 

635  is  in  a  black  space. 
(641)  just  barely  seen  <(575)- 
y — 17 


130  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEfiULA  OF  ORION. 

But  (641)  >(s67),  which  is  barely  visible,  —  i6m  of  ARGELANDER'S  scale.  There 
is  a  channel  from  635  to  663,  as  in  ROSSE. 

635  is  just  on  the  south  border  of  a  triangular  mass. 

Dark  channels  in  Regio  Picardiana. 

O,  <P,  X,  *!>,  etc.] 

Beginning  on  the  following  side  and  naming  the  dark  channels  running  approxi- 
mately north  and  south  in  order  they  are : 

i  st.  Lacus  Lassellii.     This  is  better  laid  down  on  Index-Chart  than  in  ROSSE. 

2d.  One  entering  r  on  the  preceding  side  of  SCHROETER'S  second  bridge,  and  con- 
tinuing towards  the  north  as  in  ROSSE.  (Leaving  this  order  for  a  moment  W4  con- 
nects with  W2  (this  again  verified);  W3  is  just  south  of  635.  W4  and  W2  by  no 
means  so  well  marked  as  W1  zr  lacus  Secchii.) 

3d.  The  third  channel  in  order  starts  from  the  channel  connecting  W4  with  W2 
and  runs  toward  the  north,  forming  the  preceding  boundary  of  the  triangular  mass 
(apex  to  the  north),  in  which  635  is  near  the  southern  borders.  (The  channel  con- 
necting W4  and  W2  is  tolerably  black  up  to  and  including  W2;  from  thence  it  con- 
tinues to  the  west  as  in  ROSSE,  but  is  not  so  black  after  leaving  W2.)  W1  is  the  origin 
of  another  (the  fourth)  channel  towards  the  ivest  as  in  ROSSE,  and  then  there  is  a  fifth 
to  the  north  of  c.  These  are  the  principal  ones,  and  they  are  all  nearly  exact  in 
ROSSE'S  drawing. 

A  channel  goes  from  657,  652  towards  s.  p.  as  in  ROSSK,  except  that  I  doubt  its 
crossing  the  northern  end  of  the  triangular  mass  just  described  (635  at  south  end  of 
this  mass).  If  it  crosses  this  mass  I  do  not  see  it  so  to-night,  and  certainly  the  rela- 
tive intensities  near  this  point  are  not  as  in  ROSSE.  North  of  this  (last  described) 
channel  is  another  across  q>  parallel  to  the  one  just  described  through  657  and  652,  as 
laid  down  by  ROSSE.  This  is  outside  the  limits  of  the  Index-Chart. 

506  is  a  few  seconds  south  of  the  dark  channel,  having  its  origin  in  W1  —  lacus 
Secchii,  and  516  is  still  in  the  same  bright  wisp.  From  524,  running  towards  the  west 
and  dividing  the  wisp  just  spoken  of,  is  a  dark  streak  as  in  sketch  [omitted].  [Probably 
not  due  to  contrast,  1877,  April  3.] 

Palus  Bondii. 

567  is  quite  in  the  dark  and  quite  faint  =:  16  magnitude  (ARGELANDER).  From 
575  south  of  567  and  across  to  the  southern  edge  of  c  it  is  a  very  little  brighter  in  a 
narrow  wisp,  so  that  the  space  bounded  north  and  northwest  by  c,  northeast  by  B,  and 
south  and  southwest  by  this  narrow  wisp  is  quite  dark.  It  is  undoubtedly  made  darker 
by  contrast  near  the  junction  of  c  with  B. 

i. 

i  is  an  irregular  oval  dark  mass,  separated  from  the  darker  space  just  described 
by  the  wisp  from  575  to  c  and  bounded  on  the  south  by  an  irregular  line  as  in  Index- 
Chart.  This  (i)  is  connected  to  the  dark  channel  just  north  of  K  by  a  darker  lane 
through  G)  as  indicated  on  Index-Chart.  (Sketches  made  of  the  parts  described  which 
are  omitted.)  Parallel—  17°. 8. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  131 

Pons  Schroeteri  (angle  of  position). 

Position-circle:  293°.;,  292°.8,  289°.6;  mean,  292 °.o.  ^=i75°.8  (3);  uncer- 
tain. 

Sketch  of  proboscis  near  793  (omitted). 

1877,  JANUARY  24. 

Begin  8h  30™,  end  9h  40™.     Mag.  power,  175.     Seeing  very  bad.     Moonlight. 

Using  a  pair  of  tourmalines,  lent  by  Prof.  S.  P.  L^NGLEY,  back  of  the  eye-piece 
(first  removing  the  cap  which  contains  the  eye-hole).  With  the  maximum  light  which 
passes  through  the  tourmalines,  I  see  all  four  stars  of  the  trapezium,  685,  708,  724, 
74 1)  669,  etc.  I  can  see  the  whole  of  the  Huyghenian  region  and  plainly  trace  the 
Sinus  Lamontii  between  two  bright  nebulosities  I  and  J.  No  dark  channels  seen  well, 
but  V  is  evident ;  therefore  V  is  darker  than  the  bottom  of  the  dark  channels.  The 
tourmalines  were  to-night  held  in  the  hand,  but  I  find  it  will  be  necessary  to  make  an 
adapter  for  them.  They,  however,  indicate  that  in  the  Huyghenian  region  M,  S,  R,  P, 
T,  and  between  J  and  I  it  was  the  faintest;  then  E  and  the  north  part  of  I ;  next  G, 
F,  H,  and  part  of  I,  part  of  Q  (near  N,  I  think),  are  a  degree  fainter  than  A  and  D. 
The  above  results  are  approximate  and  tentative,  and  are  not  of  much  weight. 

W1,  W2,  W3,  W4,  W5. 

[Sketch  omitted.]     >  =  "  blacker  than" ;  W,  >  W4  >  W5  >  W2. 
6 1 2  in  nebulosity,  or  very  close  to  border. 
618  inside  V. 

(642)  =:  654  each  is  just  visible. 

709  >  (641)  >  (676)  >  (567)  here  >  =  "brighter  than";  671  —  622!  —  575 
nearly,  and  589  is  a  very  little  fainter  than  671.  675  not  visible. 

A. 

589  and  622  are  correct  on  Index-Chart  in  relation  to  A.  622  is  in  a  dark  space 
half  way  from  V  to  A. 

581  >  573.     R  56  not  seen  to-night. 

L. 

L,  from  621  to  60 1,  and  from  thence  to  595,  that  is,  the  north  [shore],  is  very 
bright ;  almost  as  bright  as  A  near  it. 

I. 
No  nucleus  (602)  seen  in  I  to-night. 

F. 

X  and  F  seem  to  be  almost  separated  by  a  fainter  streak  nearly  in  the  parallel. 
The  following  end  of  F  extends  further  east  than  in  the  Index-Chart,  but  [this  part]  is 
fainter  than  the  rest  of  F.  [Sketch  omitted.] 

As  often  before  remarked  the  channel  between  F  and  (G  and  H)  is  in  the  line 

685-708. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

G. 

G  seems  elongated,  and  to-night  resembles  closely  LASSELL'S  oil-painting  of  it 
(made  in  1854,)  which  is  at  the  Royal  Astronomical  Society's  rooms. 

M. 

M  is  elongated  in  the  direction  622-741  (approximately);  it  is  just  below  M,  and 
to-night  seems  similar  in  shape  to  it. 

Frons. 

The  frons  is  Convex  to  the  east  as  ROSSE  draws  it.     The  greatest  convexity  is 

near  685. 

V. 

On  the  preceding  border  of  U,  about  half  way  (?)  from  622  to  628  (half  the  4d 
of  these  stars),  I  twice  saw  a  very  faint  star  ir  (642)  as  it  is  to-night.  It  is  just  on  the 
very  edge  [of  V].  Night  transparent  but'very  unsteady. 

G. 

(See  above.)  Its  shape  was  as  in  sketch  (omitted)  brightest  at  preceding  side  and 
brushing  off  to  a  fainter  following  point. 

1877,  JANUARY  27. 

C2  Orionis  has  a  small  companion  1 5  magnitude. 

1877,  JANUARY  30. 

ioh.  Professor  LANGLEY'S  tourmalines  mounted  back  of  the  eye-piece,  new  A. 
(Mag.  power,  175.) 

[The  tourmalines  were  gradually  rotated,   and  at  each   stage   the   appearances 
through  the  tourmalines  was  noted.] 
i  st.  Totally  dark. 

2d.  Three  trapezium  stars,  685  and  708  visible. 
3d.  Four  trapezium  stars,  685,  708,  and  741  visible. 
Drawing  made  of  the  Huyghenian  region  at  this  stage. 

4th.  At  this  stage  and  through  the  tourmalines  a 
measure  of  the  position-angle  of  the  north  shore  of  the 
Sinus  magnus  with  a  bright  wire  was  made.  Position- 
circle:  259°.5.  (Parallel,  33°4.);  ^  =  44°  (i.) 

5th.  True  outline  of  frons  seen ;  E  fainter  than  G 
and  H.  Lacus  Lassellii  seen. 

This  process  was  repeated  several  times  and  a 
careful  crayon  sketch  made  [omitted],  685  and  708 
on  s.  f.  edge  of  the  frons.  741  free  from  nebulosity. 
635  and  669  seen.  635  just  plainly  visible.  A  dark 
band  penetrates  the  Huyghenian  region  from  Sinus  mag- 
FIG.  7.  Drawing  made  through  tour-  nus  ( whicn  js  much  blacker  than  this  band)  and  divides 

maline  plates.  ,  ,  , 

into  two,  one  goes  south  past  685,  the  other  west  near  0 

and  then  makes  a  little  bay  towards  the  north  as  in  PICARD'S  (1673)  drawing  [see 
Figure  4  of  this  text]. 

G,  H,  F,  and  E  ?  divide  the  two  parts  of  the  dark  band. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  133 

W5. 

(Without  tourmalines.)     W5  is  certainly  not  so  black  as  W4  and  W1.     Has  it 
changed  since  Lord  ROSSE'S  drawing  ? 
End  iob  45™      Eye  much  fatigued. 

1877,  FEBRUARY  3. 

Begin  8h  2om,  end  9h  25™.     Mag.  power,  175.     Wt.  =  2. 

(654)  and  (675)  just  barely  seen  in  moments  of  quieter  seeing.  675  appears 
to  be  east  of  the  line  of  the  frons ;  i.  e.,  in  the  dark  space  outside  of  the  Huyghenian 
region,  but  this  is  not  absolutely  certain  [and  it  is  quite  different  from  all  previous 
determinations]. 

654  is  only  rarely  visible.    The  air  is  exceedingly  transparent. 

Parallel,  33°4.  Coincidence,  64r.i37.  Difference  of  R  A.  of  9l  and  following 
end  of  Q. 

Micrometer:   49r.oo,  49T'.6o,  49r.7o;   mean,  49r-43  ;  s  rz  i4r-7i  (3)  n  +  i47"-3  (3) 
(this  is  rather  uncertain  and  too  small  rather  than  too  large)  ;  refraction  •=.  o". 
Difference  of  R.  A.  of  0*  and  following  end  of  G. 

Micrometer:  47r-55,  47r-55»  47r-5!  5  mean,  47r-54l  s  —  i6r.6o  (3)  —  -f  165".!  (3) 
refraction,  o" '. 

Using  Professor  LANGLEY'S  tourmalines — 

i  st.  (When  the  maximum  light  was  transmitted)  careful  crayon  sketch  made 
[omitted]. 

2(1.  (Diminishing  the  transmitted  light.)  The  portion  [of  the  Huyghenian  region 
south  of  the  line  608-741  has  vanished].  Along  that  line,  or  near  it,  it  is  brighter 
[than  somewhat  further  to  the  north]. 

Order  of  brightness  (through  tourmalines). 

E  <  F,  G,  H,  Q  and  I. 

1877,  FEBRUARY  6. 

Begin  8h  45m,  end  9h  50™.     Mag.  power,  175.     Wt.  =  3. 

Measures  of  Ja  with  6\ 

Wire  put  in  the  meridian  of  &.     The  meridian  of  0'  bisects  as  nearly  as  possible 
the  apex  of  E.     The  north  corner  of  E  precedes  this  meridian  a  few  seconds, 
meridian  precedes  nearly  all  of  F.     Only  a  little  (fainter)  part  preceM.,,,  X  being  cu 

'V  om.)     If  the  apex  of  E  is  not  on  the  wire  through  Q'  it  does  not  precede  it  at 
least,  and  it  may  follow  it  2//-3//. 

The  preceding  end  of  G  comes  nearly  up  to  the  wire  through  9  . 
point  of  E  to.  &  this  wire  is  nearly  entirely  in  darker  portions  except  near  the 
ing  end  of  L,  where  it  cuts  off  a  little.     Coincidence  =  64r.i  i. 


134  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

G   (middle  point). 

Micrometer:  62T.6i>J  62r.84,  62^67;  mean,  62*.?!.  aJ  —  ir.4o,  aJ  =  13".  9  (3); 
all  these  measures  somewhat  uncertain. 

Point  where  s.  p.  edge  of  F  intersects  frons. 

(It  is  faintly  nebulous  here,  not  bright  as  in  the  middle  of  F.) 

Micrometer:  6ir.7o,  6ir45,  6ir.78;  mean,  6ir.64.  Ja  —  2*47,  Jot  —  24".  6  (3)  ; 
all  rather  uncertain. 

Tangent  to  tlje  following  edge  of  D. 

The  north  point  of  D  precedes  this.  The  point  of  tarigency  is  near  651.  All 
north  of  this  D  precedes  the  wire.  651  very  faint. 

Micrometer:  6ir.i3,  6or.98,  6ir.oo;  mean,  6ir.o4.     aJ  —  3r.O7,  aJ  —  30".  5  (3). 

ffo- 

ff0  is  not  well  denned  to-night. 

Micrometer:  56r.25,  56^36,  56r.39;  mean,  56*.  33.  da.  —  7^78,  4a  —  ^"  ^  (3)  ; 
all  quite  uncertain. 

675  seen  only  in  the  evening  in  the  nebulosity.  The  low  power  always  shows 
[small]  stars  the  best,  as  often  before  noticed.  [Probably  this  indicates  that  many  of 
the  smaller  points  of  light  are  not  true  stars  but  nebulous  nuclei  or  groups  of  small 
stars.] 

[See  1876,  March  14  and  March  22.] 


s  nucleus  near  793. 

I  cannot  see  it.  My  former  description  [examined  and]  confirmed.  The  neigh- 
borhood of  793  is  glistening  [with  minute  but  indefinable  points  of  light],  but  no  one 
point  can  be  selected  following  it. 

I  see  (but  just  see)  my  former  star  preceding  793  and  40"  (est.)  south  of  it. 

1877,  FEBRUARY  7. 

Begin  9h  om,  end  ioh  om.  Mag.  power,  175.  Wt.  zz  1-2;  parallel,  33°4;  coin- 
cidence, 64r.io8. 

Measures  of  JS  with  9'  . 

South  edge  of  following  point  of  G  [sketch  omitted]. 

Micrometer:  56r.$i,  56r43,  56r.6i,  56r.66;  mean,  56r.5o.  J6  —  f.6i,  JS  —  75".7 
(4)  ;  refraction,  zero. 

From  lacus  Lassellii  east  to  Spitze  the  south  shore  of  o  is  concave  to  the  south. 

Q  (follotving  point}. 


(g0  and  D  :  The  seeing  is  too  bad  to  measure  4d  of  these  points.) 
Micrometer:    67r.i9,    67r.39,    67r4o,    67r.i8;    mean,  6f.2g.     Jd  —  3r.i8,  Jd 
6  (4). 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  I35 

Gr  (midale  point). 

Micrometer:    70^83,    ;or.66,    70^85,    7or.69  ;    mean,   70^76.     Jd  -  6r  6<    Jd  - 

66".2  (4). 

Brightest  part  of  F  (ivhich  is  not  X). 

Micrometer:  74r.4o,  74r-42,  74r-32,  74r-34l  niean,  7^.37.  Jd  -  ior.26,  J6'  - 
I02//.o7  (4);  refraction  -f  o".o6,  Ad—  102".  i  (4). 

North  point  of  Sinus  Gentilii 

Micrometer:  70^77,  7or.9i,  70^73,  7or.8l  ;  mean,  7or.8i.  Jd  —  6*70,  J6'  — 
66".  65  (4)  ;  refraction  -f  o".O4,  -^  =  66".  7  (4). 

All  the  above  measures  are  somewhat  more  uncertain  than  usual  on  account  of 
unsteady  images. 

Frons. 

The  from  is  convex  towards  the  east.  Its  outline  is  furthest  east  near  the  par- 
allel of  685.  The  following  side  of  E  is  nearly  a  straight  line  ;  the  following  side  of 
F  is  inclined  somewhat  to  the  prolongation  of  the  following  side  of  E.  From  the  apex 
of  the  curve  of  the  frons  (near  685)  the  bounding  line  extends  to  about  half-way 
between  685  and  708  (in  R.  A.)  whenvit  meets  the  prolongation  of  the  following  side 
of  E  again  and  continues  on  this  line  to  the  termination  at  Q  [see  Index-Chart]. 

In  spite  of  the  unsteadiness  SCHROETER'S  second  bridge  is  well  seen  to-night, 
best  defined  on  the  following  side. 

654  not  seen.     g0  not  stellar  in  appearance. 

1877,  NOVEMBER  20. 
Begin  uh  25m,  end  nh  55"".     Eye-pieces  175  and  400.     Images.     Wt  —  2 

Measures  of  4d. 
Lacus  Secchii  (center). 
'     —  69".  74  (4)  with  175. 


Kefr.,  0.05 


JS      —  69".8    (4) 

ff* 

=41  ".6     (2)  with  400. 

D  (north  point). 

'     —  80". 98  : :  (2)     This  point  is  not  well  seen  to-night;  it  appears  to  be  curved 
Refr.,  0.05  towards  the  preceding  side. 


Jd      —  8i".o 

Q  (following  point). 
Jd      —  25". 67  (3)     This  appears  to  bisect  the  following  point  of  Q. 


[36  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

G  (brightest  part). 
Jd'     -  60". 88 
Refr.,        o".O4 


—  6o".09 

Seeing  very  bad  and  satisfactory  measures  impossible.  All  the  above  are  very 
poor. 

1877,  DECEMBER  2. 

« 

i  ib  30°*  to  i3h.     Drew  on  chart. 

1877,  DECEMBER  3. 
D. 

Begin  9h  30™,  end  ioh  i5m.     Wt.  =  i. 

The  shape  of  the  following  edge  of  D  is  correct  in  the  Index-Chart. 

The  brightest  part  of  D  is  within  this  edge.  There  is  no  stellar  appearance  to 
this  brightest  part. 

647  seems  to  be  in  a  bay,  whose  shape  is  (to-night)  more  nearly  circular  than  in 
Index-Chart. 

651  appears  (to-night)  slightly  preceding  the  edge  of  D. 

"W"i  .....  W5  as  drawn  on  chart.     [Drawing  omitted.] 

1877,  DECEMBER  7. 

End  i2h  14™.     Mag.  power,  400.     Wt.=  3. 

E  (south  point). 
Measures  of 


'     —        i3i"<3    (0-     This  is  the  extreme  point  towards  the  north  which  could 
Refr.,  .07  be  taken  [as  the  vertex  of  E]  south  of  this  nebulosity 

is  fainter. 


Jd'     -       i43//.45     (4) 
Refr.,  .08 


Jd     =  -i43"-5 

1877,  DECEMBER  7. 

A  (south  point). 
Jd  —  —  3 1 ".9  (2)  not  very  certain. 

W1  (lacus  Secchii). 

Jd'     =   67".65     (3) 
Refr.,  .03 

Jd  —  -}-    67.7 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  137 

1877,  DECEMBER  12. 
Beg-in  i  ih  30™,  end  i  2h  42™.     Eye-piece,  600  A,  400  A.     Wt,  —  3. 

Measures  of  4d  with  0' . 
A  (south  point). 

iih30m.     Micrometer-          67.17 

67.38 
67.48 
67.40 
67.09 


67.30 
Zero      -     -     -     64.16 


A. 

The  dark  space  which  includes  622  and  625  on  Index-Chart  is  quite  black, 
blacker  than  the  channel  preceding  A  and  separating  it  from  L.  622  is  in  the  black 
channel. 

L  (north  point}. 

'  ~  >      This  is  in  the  same  4$  as  671  and  as  622. 

66'64  I      A  little  less  Jd  than  622. 
66.70  $ 


66.71  ^  =  2r.55  =  -25".37(4) 
(602)  not  seen  ;   589  >  567. 

Tangent  to  the  north  side  of  the  curve  in  which  A  joins  B. 
6 1. 1 2,  .10,  .or,  .14;  mean,  61.09  ^8  —  3r-°7  =  3°"-54  (4) 

The  point  of  tangency  is  marked  on  the  chart. — [N.  B — Only  on  the  MS.  chart 
employed.] 

B. 

B  is  much  fainter  south  of  the  parallel  of  &  than  north  of  it ;  and  it  seems  hardly 
to  reach  (as  a  bright  mass)  the  star  575. 

9* 

I2h  7m.     4dj  60.0 r,  .19,  .23,  .01  ;  mean,  60.11,  Jd  —  4.05  =:  4o".29  (4) 

1)  (north  point). 

The  north  point  of  Dis  not  well  enough  defined  to  measure;  it  is  drawn  on  chart. 
APP.  V— 18 


138  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

South  shore  of  6  near  Spitze. 

55.69::  (shore). 

56.05  ^ 

56.40  >  south  side  of  Spitze. 

56.05  ) 
56.12 


56.16     Jd'  —  -8.00  :=  +  79".  5  9  (4)         The  following  point  was  not  clearly  seen 

Refr.,  .05  and  these  measures  refer  to  the  46  of  the 

brightest  point,  and  are  a  .little  greater  than 


—  -f-    79-6  the  Jd  of  the  south  edge,  as  it  would  be 

seen  under  the  best  conditions. 
This  south  shore  is  concave  toward  the  south. 

Q  (bisecting  the  following  point}. 
67.01,  .30,  .16;  mean,  67.16  ^#  —  3.00=  29".  85  (3) 

E  (extreme  south  point). 

79.72,  79.86;  mean,  79.79  48  —  15.63  =  15  5".  49  (2) 

South  point  of  F  (in  frons). 

75-93,  75-945  mean,  75.94  A#  —  1  1.7.8  =      i  i?".i9  (2) 

refr.,  .07 


Extreme  north  point  of  F  (X). 


73-75,   73-63;    mean>   73-69-     ^d—    9-53=       94"-8i  (2) 

refr.,  .05 


--    94".9  (2) 

When  the  micrometer  wire  is  set  at  73.63  (that  part  of  it  following  X)  it  is  all  in 
the  dark  channel.  It  just  intersects  the  frons  at  the  south  end  of  H.  Zero,  64".  155  (3) 

1877,  DECEMBER  14. 
Begin  i  ih  om,  end  i2h  40™.     Eye-piece,  400.     Wt.  =z  2. 

n- 

Just  south  of  570  there  is  a  part  of  77  much  darker  than  the  rest.  The  line  570- 
666  is  approximately  the  south  border  of  rj. 

On  this  line  about  20"  preceding  666  there  is  perhaps  two  or  three,  and  pretty 
certainly  one  small  star.  This  whole  region  preceding  666  seems  occasionally  to 
glisten  with  small  separate  points. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  139 

ft  and  K. 

The  line  573~575  is  almost  exactly  the  following  boundary  of  ft  and  K.  (602) 
not  seen. 

Channel  between  I  and  E. 

The  north  border  of  the  channel  between  I  and  E  prolonged  is  very  approxi- 
mately tangent  to  the  n.  p.  end  of  F ;  i.  e.t  X. 

I. 

The  s.  p.  corner  of  I  projects  beyond  (L  e.,  preceding)  the  general  line  of  the 
occiput.  (See  sketches.) 

L. 

The  preceding  edge  of  L  near  60 1  (star  not  seen)  prolonged  would  pass  through 
the  s.  p.  corner  of  I. 

Other  notes  placed  on  drawing. 

1878,  JANUARY  3. 
/ 

A  cap,  reducing  the  aperture  to  3.50  inches  was  put  on  the  telescope  and  eye-piece 
175  used. 

iob  30™.  The  sky  is  very  hazy,  so  that  fourth  magnitude  stars  are  not  visible  to 
the  naked  eye.  In  spite  of  this  I  see  four  stars  in  the  trapezium. 

619  and  628  very  easily,  624  and  640  easily,  and  640  is  very  little  easier  to  see 
than  624. 

ioh  40™.  685,  708,  741,  and  not  724,  570,  and  not  669. 

No  other  star  seen  except  737,  and  no  definite  outline  to  the  nebula.  The  ques- 
tion of  HOOKE'S  observation  of  1666  requires  another  arid  a  clear  night  to  settle  it. 
ANDERSON  also  saw  the  four  stars. 

i  ih.  Sky  extremely  thick,  and  large  stars  have  halos  to  the  naked  eye. 

1878,  JANUARY  5. 

9h.  Aperture,  3.50  inches.     Mag.  power,  175. 

The  sky  is  clear,  but  very  unsteady.     The  following  stars  seen : 

619,  624,  628,  640;  all  four  stars  of  the  trapezium.  Also  685.  708,  741,  724; 
also  570  and  523. 

669  and  635  well  seen.     [635  was  not  seen  by  HUYGHENS  either  in  1656  or  1694.] 

There  appears  to  be  a  star  between  635  and  0',  too  faint  to  fix  in  position,  but 
probably  647  and  651  seen  as  one. 

734,  781  (faint)  and  848;  449  and  479,  also. 

Comparing  with  HUYGHENS'  drawing  of  the  nebulosity.  In  fact,  the  frou*  suul 
occiput  are  well  seen  and  the  angle  at  E.  It  is,  however,  faint.  685,  708,  741  an- 
seen  free  from  nebula.  Siuu*  Gentilil  and  Sinus  mu<inns  very  plain.  The  stars  laid  down 
by  HUYGHENS  (1656)  are,  in  order  of  R.  A.,  523,  570,  619,  628,  640,  669,  685,  708,  724, 
74i,  734.  and  781.  The  last  is  quite  faint  and  is  out  of  place  in  HUYGHEN*'  drawing. 


I4O  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION, 

In  HUYGHENS'  drawing-  of  1694  the  stars  are  the  same  with  the  addition  of  624. 
[HUYGHENS  had  probably  seen  HOOKE'S  remark  that  the  three  stars  were  in  reality  five, 
since  that  was  published  in  1666]. 

The  orighter  nebulosity  seems  to  end  about  669.  The  folloiving  point  of  Q  is 
about  in  the  R.  A.  of  708. 

The  whole  surface  of  the  Huyghenian  region  is  mottled  much  as  in  W.  C.  BOND'S 
MS.  drawing- of  1848  reproduced  in  Annals  Harvard  College  Observatory,  vol.  v. 

ioh  30™.  Aperture,  26  inches.     Power,  175. 

Very  clear,  but  unsteady.  The  large  stars  are  blurred  even  with  1 75,  and  the  5th 
and  6th  stars  are  just  clearly  outside  of  them.  With  400  A  stars  too  much  blurred. 

675  is  pretty  steadily  seen  just  following  H ;  i.  e.  outside  of  the  Hmjglieman  region. 
There  is  no  doubt  of  this  [although  it  was  known  at  the  time  to  disagree  with  former 
observations].  In  the  place  of  LASSELL'S  I  I  seem  to  see  something  stellar ;  and  it 
even  seems  double.  This  is  the  second  time  I  have  seen  such  an  object. 

The  first  time  is,  I  believe,  not  recorded,  as  I  was  not  sure.  Nor  am  I  sure 
to-night.  [I  have  looked  many  times  for  this  object,  and  have  in  general  failed  to  see 
it]  622  is  certainly  in  the  dark  channel  foUo/riiif/  A  612  >  618.  612  is  just  on  the 
edge  of  the  nebulosity,  inside  of  U.  618  is  in  V.  602  exists,  I  think,  and  is  a  little 
out  of  position  on  my  Index-Map.  654  not  visible. 

567  about  as  faint  as  it  could  be  and  still  be  seen.     686  and  688  not  seen. 

709  is  on  the  following  edge  of  the  dark  space,  between  708  and  685. 

675  again  seen  very  close  to  the  edge  of  from. 

Just  south  of  E  near  666  are  certainly  some  very  small  stars  [or  points  of 
nebulosity]. 

The  small  star  h  in  the  channel  following  D  looked  for  and  not  seen.  End  n1'; 
seeing  unsteady,  but  quite  clear. 

1878,  JANUARY  6. 

Aperture,  3in-5O;  and  mag.  power,  175.     Sky  clear  but  not  particularly  steady. 

With  400  A  I  see  easily  the  four  stars  [of  the  trapezium]  and  can  fancy  I  see  the 
5th  and  6th  star  at  intervals,  as  I  know  exactly  where  to  look  for  them. 

With  175  I  also  thought  some  traces  of  the  5th  and  6th  star  were  to  be  seen,  but 
I  am  sure  that  these  stars  would  never  have  been  seen  by  HOOKE  if  only  of  their 
present  brightness.  647  and  651  not  seen.  End  i2b.  Windy. 

1877,  JANUARY  7. 

Photometer. 
Brightest  part  of  D  (see  observations  January  12). 

I.  II. 

9h  om  7.20  6.40 

6-85  6.55 

6.70  7.20 

Altered  mirror.  6.95. 

6.60 

6.92  (3)  6.74  (5) 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  141 

Brightest  part  of  A;  in  parallel  of  624. 

5-55: 

6.2O 

545 
ioh  4om.  6.00 


5.80  (4) 
The  brightness  of  D  is  proportional  to  — i-™  and  T^-y  from  the  construction 

of  the  photometer.     A  oc-^L^.     Hence  the  light  of  D  expressed  in  units  of  A  is 

(5.8o)2  (&*>) 

TZ~ — Ng,  etc.     Hence  from  to-night's  observations 

Dzr  0.70  A  (3) 
D  =  0.74  A  (5) 

These  measures  are,  as  yet,  only  experimental. 

A  =  1.42  D  (3) 
A  =i.35D  (5) 

1877,  JANUARY  12. 

^Photometer. 
Brightest  part  of  D  (about  midway  from  642  to  635  and  following  that  line) 

ioh  5om  4.70 

4.68 

4.10 

4.68 
ii     o  4-7° 


10  55  m.  t.  4-57  (5) 

N  B.  —  A  different  combination  of  glasses  was  used  from  that  employed  January  7, 
and  these  were  found  to  be  too  dark  to  measure  A  with. 

Brightest  part  of  E  (center  of  mass). 

nh  iolu  5.17 

4.90 
4.27 
4.80 
4.60 


ii    1-Sm.fc  4.8.  (6) 

Lamp  burned  out. 

These  measures  are  still  experimental.     All  the  adjustments  are  not  thoroughly 


settled  yet. 


142  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1878,  JANUARY  16. 

9h~9b  3Om-  Moonlight  too  strong  to  allow  of  measures  with  photometer.     Several 
trials  show  this. 

V,  6 1 2  and  6 1 8  sketched.     [Sketch  omitted.] 
ioh  om.     Eye-piece,  175.     Wt  —  4.     Moonlight. 
654  just  suspected. 

675  just  surely  seen  following  the  line  of  the  frons. 
Gr  and  H  (to  .the  eye)  both  brighter  than  F. 

Occiput. 
Zero—  123°.  7. 

Position-circle:  343c-5>  343°-2>  344°-2;  mean,  343°-6;  #=140°.!  (3) 
This  is  the  best  tangent  from  the  south  point  of  J  [LIAPONOFF  DJ  to  the  south 
point  of  E.     It  cuts  off  Y. 

Frons. 

Position-circle:  72°.o,  7i°.9,  72°. 6,  72°.9;  mean,  72°4;  ^  —  51°. 3  (4) 
This  is  the  best  tangent  to  the  whole  line  from  E  to  Q. 


r. 


Length  of  frons,     45.05  84.00 

45.10  83.68 

84.21 


45.08  83.97,  2  5.  —  38r.89,  5.  =  i9r.45,  s.  =  193" '.5 

These  measures  were  made  for  comparison  with  LIAPONOFF  [HN. — L  —  -f-  3"-7]- 

J  and  B  (preceding  edges). 

Position-circle:   n6°.8,  115°. 7,  H5°4;  mean,  n6°.o;  p=70.j  (3) 
These  measures  make  the  edge  pass  through  575. 
567  just  visible  and  a  little  brighter  than  709. 
V  and  6 1 2  and  6 1 8  are  right  in  Index-Chart. 

Dark  channel  between  E  and  F. 

Position-circle:    342°. 5,    338®. 8    (half  weight);    342°. i,    good;    mean,   341°. 6; 
_p-i42°.i  (3) 

Dark  channel  between  I  and  E. 

Position-circle:   258°^  — 226°   (i)    uncertain. 

Pons  Schroeteri. 

:ih.  Position-circle:  3  14°. 2,  315°. 6,  318°. 7;  mean,  3 16°.2  ;  p  =  167°. 5  (3)  uncer- 
tain.    All  three  measures  poor.     (654)  has  not  again  been  seen. 

D. 

The  north  point  of  D  does  not  seem  sharp  to-night,  and  I  believe  I  have  noted 
this  before.     This  deserves  attention  in  a  dark  night.     [See  1877,  Nov.  20,  Dec.  12.] 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


H3 


1878,  JANUARY  23. 

Set  micrometer  Mare  on  the  parallel  through  &  —  628. 

From  628  to  the  R.  A.  of  g0  this  line  is  within  the  nebula.  From  628  to  the  R. 
A.  of  651  this  line  is  in  nebulosity  fainter  than  the  surrounding  nebulosity.  This  line 
is  very  little  south  of  the  south  border  of  T.  It  is  immersed  in  the  nebulosity  of  R ; 
following  R  it  passes  through  a  dark  space  (on  the  map)  and  is  finally  involved  in  //. 
Beyond  (i.  e.,  following]  R  it  does  not  intersect  the  Huyghenian  region  proper. 

Preceding  628  this  line  passes  a  little  north  of  567.  Half  way  from  this  line  to 
575  is  about  the  point  where  B  ceases  to  be  quite  bright,  and  where  it  begins  to  be 
nearly  faint  up  to  575.  [This  is  different  from  ROSSE,  1867.] 

W1. 

48  of  center. 

57r.i7,  .37,  .22,  .30,  .40,  .55,  .34;  mean,  57^34 

Zero,         64. 1 5 


=    6r.8i  =6  7".  7 
Refr.,  0.3 


+  68".o 

The  parallel  through  685  cuts  off  X,  and  passes  (as  exactly  as  I  can  see  to-night) 
through  5.  /  corner  of  I.     [.  • .  J8  =>_  95".'8,  G.  P.  B.] 

Spitze. 

The  s.  f.  point  of  G  is  about  on  same  parallel  with  n.  p.  point  of  D  and  with  center 
of  Wlt 

Near  793  in  p  =.  i9O°-2oo°,  s  —  3o"-4o"  is  a  faint  star  which  I  have  noted  [18761 
March  14  and  22,  and  1880,  January  3].     Sketch  (omitted). 

F,  G,  H,  >  I  or  E. 
There  are  also  two  surfaces  in  P?  and  Q  which  are  brighter  than  either  I  or  E. 

End  ioh  iom. 

1878,  JANUARY  24. 

nh45m.     Eye-piece,   175.     Wt='2. 
709  not  brighter  than  666. 
66 1  >  676. 

1878,  JANUARY  26. 

A. 

Photometer. 


Washington 
Sid.  Time. 

Readings. 

Notes. 

h.   m. 
4     53 

3-95 

The  part  A  measured  was  that  following  619.     Measures 

4-85 

not  easy.     Lamp  flaming. 

3-72 

Altered  mirror. 

4-3=> 

Good. 

5       7 

3-90 

t=s      o 

Mean  4.14 

144 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


In  future  I  shall  take  D  (that  part  previously  used  and  defined  by  the  line  635- 
647)  as  the  unit  of  brightness,  as  the  position  of  these  stars  enables  me  to  define  the 
surface  used  for  comparison  readily.  I  notice  that  much  of  the  uncertainty  of  readings 
is  to  be  attributed  to  the  difficulty  of  selecting  the  same  surface  for  each  comparison. 

N.  B. — In  what  follows  the  brightest  part  of  each  mass  is  used,  unless  otherwise 
mentioned. 


Washington 
Sid.  Time. 

Readings. 

Notes. 

h.     m. 

D. 

5       7 

4-30 

5.10 

4-45 

4.72 

5     15 

4.28 

5     li 

Mean  4.57 

5     15 

5-35 

E. 

5.08 

4.70 

4.80 

• 

5      20 

5.08 

5     18 

Mean  5.00 

F. 

5      20 

3-88 

The  lamp  was  flaming  at  first,  so  that  much  of  the  light 

4-52 
4-43 

came   from   the   burning   wick.     It   gradually  grew 
fainter  until  about  constant. 

5     27 

4.42 

5     24 

Mean  4.31 

5     28 

4.08 

G. 

4-75 

4-45 

5     34 

4.28 

Good. 

5     31 

Mean  4.39 

D, 

5     36 
5     4i 

4.08 
4-45 
4-50 
4-50 

This  measure  of   D  satisfactory  ;  and   it  shows  the  lamp 
to  be  fainter  than  at  first,  since  it  has  to  be  nearer  to 
the  mirror.     The  first  D  measure  was  also  good,  but 
not  as  good  as  this. 

5     39 

Mean  4.38 

r. 

5     43 

6.55 

The    part   used    is   on   the    line  708   to   the    point  of   Q 
and  the  width  of  the  mirror  [15"]  south  of  Q.     Eye 
very  tired. 

6.15 

7-50 

5     57 

7.78 

- 

5     50 

Mearfj.oo:  : 

MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Summary. 


'45 


t 

Mass. 

Reading. 

h. 

m. 

5 

o 

A                          4.14 

ii 

D                          4.57 

18 

E                         5.00 

24 

F 

4.31 

3i 

G                          4-39 

39 

D                           4.38 

5 

50 

fi 

7.00  :  : 

These  measures  are  reduced  for  each  value  of  D  separately. 


I.    D  =  4.57- 

II.  D  =  4-38. 

A 

.  Mean  I  and  II. 

A  =  1.22  D 

A  =  1.  12  D 

0.10 

A  =  1.17  D:* 

E  =  0.83  D 

E  =  0.77  D 

.07 

E  =  0.84  D 

F  =  1.  12  D 

F  =  1.03  D 

.09 

F  =  1.07  D: 

G  =  i  .  08  D 

G  =  i.ooD 

.08 

G  =  i.  oo  D 

fi  =  0.43  D 

fj.  =  0.39  D 

0.04 

H  —  0.41  D  :  : 

1878,  JANUARY  28. 

gh  ^m  m   {  beg-in  ;   ;h  6m  sid.  t.  end.     Mag.  power,  400.     Windy      Wt. 
At  8h  54™  654  just  visible  =  618.     675  not  seen. 

Measures  of  Jd  (zero  —  64r.i53). 

E  (south  point); 
Mic.  78^93,  78.94,  78.96;  mean  78.94  (3)  J6f  —      i4r.79 


zr  4. 


refr. 


.08 


,  sid.  t. 


F  (brightest  part). 


Mic.  74r.47,   74.62,  74.48,   74.41;  mean,  74r495  (4)  *#  -  ior.34=  iO2//.86,  refr. 
.06;  Jd  -~  i02//.9  (4) 

G  (south  point). 


Mic.  7ir.6i,  71.59,  71.93,  7I-9°I 


refr., 


.04 


(4) 


•This  part  of  A  is  not  that  previously  used.     It  is  to  be  remarked  that  the  same  zero  for  D  (4.5: 
to-night  and  January  12. 

A  PP.  V 19 


146  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

N  (south  point  in  frons). 

Mic.  7ir.6i,  71.59;  mean,  7ir.6o;  46'  —  7*45  zz      74". n 

refr.,  .04 


—  _  74".2     (2) 
I  (bisecting  the  preceding  angle  of  I,  near  A^). 

Mic.  7ir.93  46'  zz  7.78  zz  77//4o;  refr.,  ."04;  46  —  —  77^4  (O 

Gr  (brightest  parf). 
Mic.  7or.67,' 70.94,  70.87;  mean,  70^83;  46'  —          6.68 

refr.,  .04 

—~-  '0        \O  / 

Star  654. 

46    Mic    63.356  4a  Mic.  60.96 

.29     star  extremely  faint.  61.10 

63.29  60.82 


63.31  60.96 

—        0.84  4  a   zz         3.19 

46     =  +  8".36  (3)  4a  =     3i".74  (3) 


G.  P.  BOND  gives  the  co-ordinates  for  1870.7  4  a.  zz  +  32"-6         46  zz  -f  9/7.2. 
HOLDEN-BOND  (in  a)  zz  —  o/7.9  ;   (in  6)  zz  —  o".8. 

D. 

D  has  to-night  no  sharp  bright  north  point  upon  which  to  measure  the  46.  It 
has  been  so  during  this  whole  opposition,  and  I  believe  this  is  quite  different  from  the 
appearances  in  i874-'75~  '76.  The  north  part  of  D  is  uniform  in  tint,  i.  e.,  the  extreme 
north  point  is  not  suddenly  bright.  The  air  very  clear  and  images  good.  Drawing- 
made  of  D  [omitted]  in  which  the  surface  h  (see  Index-Chart),  once  called  s  star,  is 
represented  as  about  in  the  center  of  the  triangle  635-669-651.  It  is  really  a  little 
north  of  the  center  of  this  triangle.  647  is  shown  within  a  dark  bay,  but  further  within 
than  formerly  drawn.  The  darkness  extends  an  equal  distance  from  647  towards  the 
west,  north,  and  south,  h  is  as  bright  as  the  neighboring  part  of  D,  on  the  line  joining 
h  and  647  and  two-fifths  of  the  way  from  h. 

South  shore  of  G.     (Spitse.) 

46.  Mic.  55r.88,  56.06,  56.16,  56.06;  mean,  56.04  .  46'  zz          8r.n 

-        8o".68 
refr.,  .05 


=  +    So".  7     (4) 


MONOGRAPH  OF  THE  CENTRAL  F*ARTS  OF  THE  NEBULA  OF  ORION.  147 

Measures  of  4  a  folloiv. 

Spitze  (G). 
Mic.  47r.83,  47.74,  47.80;  mean,  47r-79;  4<*?  = 


=  +i62".75 
refr.,  -f          .01 


=  +    i62".8     (3) 
Q  (following  point). 

Mic.  48r.62,  48.92,  49.02,  48.68;  mean,  48.81;  Ja—         i5r-34 

4  a?  =  +152".  60  (4-) 
refr.,          -f         -°2 


#<,  (MO  nucleus  visible,  brightest  part  taken). 

Mic.  56r.37,  56.50,  56.48;  mean,  56r45l  ^«'  =         7-7° 

Ja  =  +  76".6    (3) 

This  is  in  same   4  ex.  as  676,   which  is 
according  to  Gr.  P.  BOND  for  1877.0  v  JOL  —  -{-  77".  9 

HOLDEN-BOND,  —  —      I  ".3 

F  (brightest  part). 
Mic.  6ir.32,  61.60;  meaw,  61.46;  z/«x  =          2.69 


N.B.     G  >  F. 
G  (brightest  part). 

Mic.  62r.46,  62,78,  62.41;  wzertw,  62.55;  ^a>  —          l-6° 

=      i5".92 
^«  =+i5y/-9    (3) 

D  (following  edge). 

7b  8m  sid.  t. 

Mic.  6or.i7,  60.20,  60.18;  mean,  6or.i8. 

—  2.97,  4a  =  +  297/.55     (3).     [Error  of  i  revolution;   mean,  61.18.] 


On  G.  P.  BOND'S  MS.  (unpublished)  drawing  of  1861,  Mnirh   n,  SCHROETER'S 
second  bridge  is  indicated. 


148  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

709. 

In  my  drawings  and  notes  I  have  placed  709  exactly  on  the  following  edge  of  the 
dark  space  between  685  and  708,  or  exactly  on  the  preceding  edge  of  what  G.  B.  BOND 
described  as  the  cometic  tail  to  708.  In  Lord  ROSSE'S  drawing  it  is  included  within 
this  cometic  tail,  and  on  several  of  G.  P.  BOND'S  elaborate  drawings  (lent  me  by  the 
kindness  of  Prof.  E.  C.  PICKEKING,  director  of  Harvard  College  Observatory),  it  is  in 
exactly  the  same  position  as  ROSSE,  1867.  I  am  confident  that  it  is  not  so  now,  but 
I  will  again  examine  it. 

Night  very  clear  and  images  good. 

Zero,  64^153  (3) 

1878,  FEBRUARY  4. 
Photometer. 

E  (brightest  part). 
Sid.  t. 
6h  43m;  6in.oo,  5.70,  6.00,  6.30;  mean,  6.00.     [Flame  too  high.] 

D. 

[I  intended  to  choose  the  same  part  of  D  as  formerly  used,  i.  e.,  on  line  647-635 
and  about  10"  north  of  647,  but  I  was  not  as  successful  as  I  desired.  The  third  reading 
particularly  was  on  a  surface  from  15"  to  20"  preceding  the  part  used  before] 

6h  5im-  4-35,  4-55  [5-oo],  4-50;  mean,  4.47  (5) 

Flame  still  too  high,  but  a  little  lower  than  for  E. 

D. 

t 

7h  3m;  3-9°»  3-9°)  3-8?;  mean,  3.89.     Lamp  lower. 

The  part  of  D  used  here  (and  above)  was  in  the  line  of  642  and  647. 

G. 

7h  nm;   3.78,  3.68;  mean,  3.73.     Lamp  lower  a  very  little. 

The  bright  mirror  of  the  photometer  is  a  semi-circle  with  a  diameter  of  about  1 5' 
on  the  edge,  i.  e.,  about  0.027  inches. 

Removed  photometer.     Eye-piece  1 75  put  on.     Air  beautifully  clear,  but  unsteady. 

675  just  seen,  about  on  edge  of  the  frons  with  this  power. 

V?7  LASSELL'S  1.     Something  is  certainly  in  this  place.     I  should  say  it  was  a 

double  star. 

654  and  602  looked  for,  and  neither  seen.  (New)  small  star  between  a  and  709. 
I  am  not  thoroughly  certain  of  it,  as  I  do  not  see  a  satisfactorily.  ANDERSON  sees  it. 
End  ub. 

[The  reduction  of  the  photometer  measures  gives  E  —  0.55  D.  This  is  doubtful, 
for  the  reasons  given  above.  G  —  1.09  D.  This  again  is  not  thoroughly  satisfactory.] 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  149 

1878,  FEBRUARY  5. 

Photometer. 

E. 

6h  28ra  sid.  t.  9.10,  9.08,  8.83;  mean,  9.00.  The  photometer  has  been  slightly 
changed,  so  that  more  rays  fall  on  the  screen.  The  change  consisted  in  making  the 
holes  through  the  body  of  the  tube  a  little  larger. 

D. 

6h3im;  7.77,  8.08,  8.50,  8.35;  mean,  8.18.  The  nebula  is  so  far  beyond  the 
meridian  that  the  part  of  D  which  I  am  forced  to  use  is  not  exactly  the  same  as  that 
chosen  for  the  unit  surface,  although  it  varies  very  little  from  it. 

G. 

611  34m;  7-52>  7-63>  7-85;  mean,  7-67- 

F. 

6b  38m;   7.98,  8.57,  7.60,  7.73;  mean,  7.97. 

The  difficulty  with  these  measures  is  to  fix  upon  the  brightest  part  of  the  mass  F 
[The  same  difficulty  has  been  always  experienced  in  the  measures  of  Ja  and  Jd  of 
this  brightest  part.] 

611  45m;  948,  9.05,  9.95,  9.40,  9.41  ;  mean,  9.46.  The  measure  9.95  was  taken 
too  near  to  the  following  side  of  I. 

D. 

6h49m;  8.53,  7.79,  8.70,  8.70;  mean,  8.43. 

Q.       • 

6h  56m;  8.86,  8.88,  9.23,  8.90;  mean,  8.97.  The  surface  used  has  one  edge  in  the 
frons  in  the  line  joining  the  Spitze  with  605. 

A. 

7h  i2m;   5.28,  5.27,  [4.60::],  5.31  ;  mean,  5.29. 

A  is  a  difficult  mass  to  measure,  as  it  is  hard  to  fix  upon  the  surface  to  be 

measured. 

E. 

7b  i8m;  7.80,  8.12,  8.25;  mean,  8.06. 

D. 
7h  22m;  7.00,  6.90,  6.72,  7.06;  mean,  6.92. 

G. 
7h  28m;  7.05,  7.04,  7.27;  mean,  7.12. 


150 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


The  images  are  quite  tremulous ;  but  the  photometer  is  working  very  well,  as  I 
have  a  more  certain  adjustment  for  making  the  beam  of  light  from  the  lamp  move 
along  the  axis  of  the  rod  than  that  formerly  used.     The  lamp  was  satisfactory  through 
out,  but  continuously  diminished.     It  was  certainly  lower  to  the  eye  after  f\ 

The  rough  reduction  of  the  photometer  measures  gives : 


Sid.  Time. 

Mass. 

D  =  8.18. 

0  =  8.43- 

h.  m. 
6  28 

E 

0.83 

0.88 

31 

D 

34 

G 

1.14 

1.21 

38 

F 

1.05 

I.  12 

45 

I 

0-75 

0.79 

49 

D 

6  56 

Q 

0.83 

0.88 

D  =  6.92. 

7  12 

A 

i./i 

18 

E 

0.74 

22 

D 

7  28 

G 

0.94 

1878,  FEBRUARY  20. 

7h  6m  sid.  t.     Eye-piece,  1  75  A.     Wt.  —  2. 
(The  images  are  too  unsteady  for  400.) 


Mic.  76.27;  zero,  64.15. 


south  point  of  F. 


—  12.12  —  120".  5  7 
*    refr.,  —          .07 


(i) 


F  extends  to  the  south  beyond  this,  but  it  is  so  faint  and  uncertain  a  boundary 
to-night  that  measures  should  not  be  made  on  it.  It  is  easily  seen,  however,  in  the 
absence  of  bright  wires. 

south  point  of  F. 

M1  —      11.54 
=  ii4".8o 
refr.,  —          ,07 


.  75.77,  75.54,  75.75;    mean,  75.69. 


=n4".9     (3) 

These  measures  refer  to  the  well-defined  and  brighter  southern  edge,  which  is, 
however,  within  the  mass  of  F.  654  suspected;  675  invisible;  709  barely  visible. 
Then  drew  on  large  chart  [omitted]  End  ioh  iora  m.  t.  Work  very  unsatisfactory. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  151 

1878,  FEBRUARY  25. 
Drew  on  chart;  clouds  8h  15™. 

1878,  FEBRUARY  26. 
[Drew  V,  H,  D  on  chart.] 

F  to  the  eye  seems  brighter  than  usual.     Seeing  very  poor ;  images  unsteady  and 
ill-defined. 

Photometer. 
F  (brightest  part). 

6h  om  sid.  t.  5.25  :  wt.  i 

.  4.60  wt.  2 
4.65  wt.  2 
4.45  wt.  2  Good. 


4.66  mean  by  weights. 

D. 

6h  iom ;  4.80,  4.20,  4  50  (good),  4.25  ;  mean,  4.44 ;  eye  estimated  F  >  G,  but  not 
much ;  also  G  >  H,  ditto.  These  estimates  very  difficult  [G].  (These  estimates 
belong,  not  to  the  brightest  part  of  G,  but  to  a  point  marked  on  chart.) 

6h  i5m;  4.10,  4.70,  4.40;  mean,  4.40. 

This  point  is  towards  the  following  end  of  G.  At  first  this  was  chosen  as  brightest 
part,  and  the  error  was  only  discovered  at  6h  40™  on  remeasuring  G.  This  shows  me 
that  a  chief  source  of  error  is  certainly  the  difficulty  of  settling  on  the  point  to  be 
compared.  Some  means  must  be  adopted  for  fixing  on  the-  same  point  night  after 
night.  If  this  is  not  done  the  measures  will  not  be  strictly  comparable. 

I  >  E  (eye). 

E. 

6h  22m;  4.30  (wt.  i),  4.80  (good),  wt.  2,  4.65,  wt.  2 ;  mean,  by  weights,  4.64. 

I. 
6h  29m;  4.45,  4.55  (very  good),  4.40;  mean,  4.47. 

D  (brightest  part  ?). 
6h  29m;  3.96;  this  is  a  little  s.  and  /  of  the  proper  unit.     Taken  by  mistake. 

D  (unit  surface'). 
4.40,  4.05,  4.20;  mean,  4.22. 

H. 
6b  35m;  4.10,  4.20,  4.00;  mean,  4.10. 


152 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


[G]. 


6h  40™;  4.0.     This  is  the  following  part,  same  as  at  6h  i5m. 

G  (brightest). 

3.90,  4.00;  mean,  395. 

A  not  well  enough  seen  to  measure. 

Eye-pieces,  175  and  400. 

a  near  724  seen  ;  distance  quarter  (709-724)  r=  1 2" .  709  is  on  the  following  edge 
of  a  darker  part  of  the  dark  space  between  685  and  708,  but  the  darkest  and  most 
obvious  space  between  the  two  has  its  following  edge  west  of  709,  so  that  G.  P.  BOND 
appears  to  be  right  in  his  position  of  this  star  relative  to  the  cometic  tail  to  708.  This 
requires  examination  on  a  better  night. 

U. 

Prolong  the  lines  640,  625,  and  624,  6(9  to  the  following  side  of  A;  between 
these  lines  the  channel  between  U  and  A  is  darkest;  north  of  the  last  line  (624,  619) 
it  is  brighter  for  about  10",  measured,  along  the  axis  of  the  channel,  and  then  darker 
again. 

654  not  looked  for.     675  just  barely  seen. 

h  well  seen  as  before.  LIAPONOFF'S  points  F  and  c  [near  651  and  654]  are  neither 
of  them  well  enough  defined  to  measure. 

Reduction  of  Photometer  measures. 


Mass. 

I.  D  =  4.44. 

11.  D  =  4,22. 

F 

0.91 

0.82 

[G] 

1.02 

0.92 

E 

0.92 

0.83 

I 

0.99 

0.89 

H 

1*17 

1.  06 

[G] 

0.97 

0.88 

G 

1.26 

I.I? 

Brightest  D 

1.30 

I.I? 

1878,  FEBRUARY  28. 
A>D;  F  >  G  =  H  ;  I>Eby  eye. 

Photometer. 
D. 

6h  23m;  4.90,  5.30,  4.80,  5.15;  mean,  5.04  (4) 

E. 
6h  27"';  5.95,  5.75,  5.35,  5.25,  5.40;  mean,  5.54  (5) 

F. 
6b  35ml  4-70,  4-70,  4-30,  4-55  (good);  mean,  4.56  (4) 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  153 

H. 

6h  40m;   5.25,  4.90,  5.25,  4  70;  mean,  5.03  (4) 

D. 
6h  46™;   5.65,  4.60  (good),  5.20,  5.50,  4.30;  mean,  5.05  (5) 

I. 

6h  54m;  5.10,  5.40,  5.25,  535;  mean,  5.28  (4) 

D  (brighter). 

6h  68m;  4.65,  4.75,  4.55;  mean,  4.65   (3) 
This  is  on  the  line  645-663. 

1878,  MARCH  4. 

Eye-piece,  175.     Wt.  —  i. 

8h  654,  seen;  675  and  602,  not  seen.  Drew  all  the  black  channels  on  large 
chart,  and  all  the  details  that  could  be  seen  [omitted].  Images  too  poor  for  measures. 
Too  windy  for  photometer. 

8h  30™.  Order  of  brightness. 

i.  A.     2.  D.     3.  G,  H,  F.    4.  I,  E. 

M  quite  bright ;  brighter  than  usual.     M  —  I,  El! 

1878,  MARCH  5. 

Begin  7h  iora,  sid.  t. 

Eye-piece,  400  A.     Measures  of  Jd ;  zero  =  64^13.     Wt  =  3. 

F  (south  point  preceding  frons)f 

That  is,  these  measures  refer  rather  to  the  s.  p.  edge,  which  is,  perhaps,  not  so  far 
south  as  the  part  of  F  in  the  frons. 

Mic.  75r-55>  7543i  75-6l5  mean>  75-53-     ^d'  —  n-40  =  —  "3"4i 

refr.,      —  —       0.07 

H  (south  edge). 

This,  again,  is  preceding  the  frons,  and  is  the  limit  of  the  brighter  parts  of  F  about 
in  the  meridian  of  666. 

Mic.  71.81,  72.00,  71.94;  mean>  7I-92-     4&  =  7-79  =  —  77"-49 

refr.,  —     0.05 

JS  —  77"-5 

This  parallel  is  only  a  very  little  south  of  the  south  point  of  N.     All  of  G  is  nnrtl, 

of -77"- 

APP.  V 20 


154  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

This  parallel  is  almost  the  axis  of  the  curve  of  the  following  edge  of  I,  s.  f.  602. 
It  is,  however,  a  little  north  of  the  axis  of  that  curve. 

N  (south  point). 

Mic.  71.66.     Jd'  —  7.53  =  —  74".9i   (0 
refr.,        =  —      0.04 


AS  =-75"-o    (i) 

F. 
A  faint  channel  is  visible  through  the  following  end  of  F,  as  before. 

Q. 

Drawing  of  masses  in  and  near  Q.     N,  nt  q,  etc.     [Omitted.] 

n  (north  end). 

Mic.  67.70;  ^  =  3-57  =  —  35"-52  (0 

q  (middle). 

Mic.  66.48  ;  Jd  —  2.35  —  —  23".38  (i).     Stopped  ;h  35'",  sid.  t. 

y  (north  end). 


Mic.  69.6.2,*  69.94,*  70.17,  70  22  ;  mean,  69.99.     ^8  —  5.86  =  —  58".  3  (4) 
*  Recorded,  7or  ;  if  it  really  is  7or,  as  is  possible,  the  borders  being  here  very 
uncertain,  then 

48    =-  63".37  (3) 

refr.,  ^r  —      0.04 


=—  63"3     (4) 

6  (south  end). 

Mic.  73.67,  74.27,  74.34;  mean,  74.09.     Jd  —  —  99".o8 

refr.,  —        .07 


=  -99"-'     (3) 

The  last  two  measures  pass  nearly  through  the  brightest  part  of  F.     .  •  .  JS'  —  — 
ioi".i7;  refr.  —  —  o".o7,  and  Jd  brightest  part  of  F  =.  —  ioi".2  (2). 

I  (s.  f.  corner). 

Mic.  73.93  ;  4d'  —  9.80  —  —  97".49  ;  refr.  =  —  o//.o7.     .  •  .  46  —  —  97/x.6. 

I  (corner  near  A^}. 

8h  i5m,  mic.;   72,22,  72.36;  mean,  72.29.     Jd'  —  8.16  —  —  8i".28 

refr.,  —      0.06 


=-8l".2       (2) 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  155 

This  parallel  —  81"  is  still  a  little  north  of  the  axis  of  the  curve  of  the  folloiving 
edge  of  I  near  602  ;  the  4d  —  Si".  2  for  I  near  A!  is  uncertain,  as  the  point  is  not  well 
defined. 

V  (south  point)  or  M  (north  edge). 

Mic.  66.30,  66.48,  66.44;  mean,  66.41  ;  J6  —  2.28  —  —  22".68  (3) 

refr.,  —  —      o.oi 

Jd      =-22".7      (3) 

Images  unsteady  towards  the  last.     Apparently  some  haze. 

1878,  MARCH  9. 

675  seen  by  Professor  HALL  outside  from  by  2"  ±-  The  seeing  is  very  steady. 
1}  seen  by  Professor  HALL,  but  not  double.  Very  faint,  a  seen,  a  >  675.  647  is  on 
the  following  and  north  side  of  a  channel. 

Photometer. 

E.  6.50,  6.80,  6.80;  mean,  6.70  (3) 
D.  5.90,  6.00,  5.80;  mean,  5.90  (3) 

[G].  (On  line  through  647  and  tangent  to  the  preceding  side  of  F.)     6.50  (i) 
G.  5.80,  5.70,  5.90;  mean,  5.80  (3) 
I.  5.60,  5.70,  5.95;  mean,  5.75  (3) 

Lamp  lower  I  This  note  put  in  because  the  reading  for  I  appears  so  large.  On 
looking  at  I  with  the  eye-piece  it  certainly  appears  much  brighter  than  E.  I  >  E  (eye). 

1878,  MARCH  9 

[F].  (On  line  through  676  and  apex  of  F.)     6.30,  6.60,  6.40;  mean,  6.43  (3) 

F.  5.60,  5.40;  mean,  5:50  (2) 

I.  (On  line  from  628  to  apex  )     5.90,  5.95  ;  mean,  5.93  (2) 
I  returned  to  tins  mass  to  verify  the  former  measures. 

D.  5.60,  5.40,  5.50  (good);  mean,  5.50  (3) 

E.  6.20,  6.30  [5.40:  :  rejected],  6.10  (good);  mean,  6.20  (3).     End  7h  37"'  sid.  t. 

1878,  MARCH  20. 

J  (preceding  point). 

Eye-piece  400.     Wt.  =  2. 

7h  35msid.  t.     4a!  73^69,  74r.i6,  73^96,  73^92  ;  mean,  ^3r-92 

zero,    64  .  1  5 

Ja'  —  9.78  =  97"-29 
refr.,  .01 

Ja-  97"-3    (4) 


68.86,  68.98  :  :;  mean,  68.92.     4a  —     4r-77 

=  47"4 


156  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1878,  DECEMBER  6. 

Begin  4b  25™,  sid.  t,  end  5b  35™,  sid.  t.     Eye-piece,  400  A.     Wt.  :=  i. 
Measures  of  da.;  zero  of  micrometer  64r.O9. 
I. 

West  point  —  LIAPONOFF'S  A,  approximately.  The  point  on  which  I  endeavored 
to  measure  is  in  the  straight  line  of  the  occiput  itself,  and  therefore  it  is  inside  the 
edges  of  I  which  extends  west  of  this  point  some  seconds  with  tolerably  bright  nebu- 
losity. The  bright  nebulosity  ends  about  at  this  point,  which  is  about  the  Jd  of  685. 

I. 

4b35m;  ^a'  =  70r.6i,  70.31,  7025,  70.43;  mean,  70.40. 

'  —    6r.3i  —  —  62x/.77  (4);  refraction,  -f  o".o  i. 
—  62".8  (4). 


4b  50™  ;  J  a!  •=.  68.91,  68.8  1,  69.00,  69.00;  mean,  68.93. 
Ja'  —  da  —  4r.84r=  —  48".  15   (4). 

D. 

5b  iom;  tangent  to  the  following  side  of  D. 

Jo!  —  6i.r.26,  61.2  1,  61.09,  61.10;  mean,  61.15. 
2r.94  =  +  29'\25   (4). 


Second  bridge  of  Schroeter. 

About  4h  29™;  I  could  see  this  very  well  in  spite  of  the  moonlight;  preceding  it 
as  far  as  D  the  channel  was  very  black  (the  blackest  part  of  T)  following  it  to  ff0  the 
channel  way  not  so  well  defined. 

LASSELL'S  b  (near  685). 

At  about  4b  25™  this  object  was  suspected  just  as  on  previous  occasions.  I  am 
by  no  means  sure  of  its  existence.  I  am  certain  that  there  is  some  objective  cause 
for  so  many  suspicions.  At  5b  it  was  not  to  be  seen. 

Neither  654  nor  675  are  visible. 

9* 

5b  20m;  4a!  —  56r.37,  56.28,  56.48,  56.39;  mean,  5638. 
4a!  —  da  =7r.7i  =+76".7o  (4). 

6  (following  point. 

5h  3om;  Ja'  =  47r-95,  47r-7i,  47r,83,  4^.02;  mean,  47r.88. 

Ja'  —  Ja  —  -}-  i6r.2i  —+  162".  26   (4). 
Images  very  unsteady.     Moonlight. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  157 

1878,  DECEMBER  u. 

Begin  [ih  i5m,  end  I2h  5m.     (Cloudy.)     Mag.  power,  175.     Wt=  i. 
Moonlight.     Images  too  unsteady  for  measures. 

E. 

E  is  nearly  uniform  in  brightness  all  over ;  the  n.  f.  corner  appears,  however,  a 
little  brighter.  [This  may  be  erroneous.] 

Y. 

Y  is  very  faint ;  [the  outlines  are  barely  seen]  it  is  fainter  than  the  neighboring 
parts  of  E. 

Z. 

Z,  up  to  and  including  581  and  573  is  quite  marked  by  brighter  than  the  masses 
n.  and  e.  of  it.  The  contrast  seems  more  striking  than  I  remembered  it  from  former 
years. 

F,  G,  H. 

All  of  about  the  same  brightness,  but  F  is,  perhaps,  the  brighter.     This  is  doubtful. 

F. 

The  brightest  part  of  F  is  south  of  X. 

H. 

The  axis  of  H  appears  to  be  nearly  in  the  line  685-622. 

H  and  M. 

The  line  of  the  following  edges  of  H  and  M  (prolonged)  passes  between  640 
and  624. 

P. 

676  is  in  the  dark  channel;  but  very  close  to  the  preceding  edge  of  P. 
i2h  iom;  sky  all  cloudy. 

1878,  DECEMBER  23. 

i  ih ;  67 1  >  709  ;  709  >  647,  but  not  much  brighter.  709  not  much  more  easily 
seen  than  666.  Images  very  bad.  1 2h  45™  no  better. 

1879,  JANUARY  10. 

8h-iob.     Mag.  power,  400.    Wt.  :=  5.     Prof.  A.  HALL,  observer. 

"At  about  8h  22m  I  looked  at  the  nebula  of  Orion  with  power  400  A.  The  images 
were  very  good.  I  could  see  no  star  inside  the  trapezium  where  Professor  Boss  reports 
a  companion.  On  examining  the  region  near  [685,  708,  741]  I  saw  two  faint  stars 
near  the  places  indicated  in  the  sketch  [omitted].  The  faint  stars  are  denoted  by  a 
and  6.  On  looking  again  at  9l1  40™  I  could  not  see  either,  the  seeing  not  being  s 
good,  a  and  6  were  about  of  same  brightness." 

[Note  by  E.  S.  H.— a  is  in  the  position  of  LASSELL'S  double,  "6"  (see  Index-Map) 
1)  is  from  708  in  ^  —  45°  8  =  48"  est.  from  sketch]. 


158  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1879,  JANUARY  18. 

End  nh  2om.     Eye-piece  200.     Wt.  =  i.    . 

Occiput. 

p  =  33o°.6,  331.8,  330.5,  333.2;  mean,  33ic.5 

4747 


P-  143-2  (4) 
Frons. 

p  —  244°.;,  244.6,  245.1,  247.5;  ™ean>  245°-5 

294.7 


p  =     49.2  (4) 

None  of  the  faint  stars  are  visible;   709  is  just  barely  seen;  633  and  617  not  seen. 
Images  very  bad. 

1879,  JANUARY  20. 

9b  55™.     Mag.  power,  400  A.     Wt.  —  3. 

Measures  of  JS ;  zero  —  64^13. 
Brightest  part  of  F. 

Mic.  74r47,  74rs8,  74*40,  74.37;  mean,  74^46 

48'  —  10.33  —  —  iO2".76 
refr.,  0.06 


—  I02".8     (4) 

N.  point  of  E. 
Mic.  73r.92 ;  clouds;   73r-76;  mean,  73r.84 


9h55m 


J6'  —      9.71  n  —  96". 60 
refr ,  —      0.05 

JS  -  —  96".7    (2) 

The  n.  point  of  X  (F)  is  further  north  still. 

Brightest  part  of  G. 
Mic.  7or.77,  70.64,  70.55,  70.64;  mean,  70.65 


J6'  —  —       6.52  =  —  64".86 
refr.,  0.04 


-64.9    (4) 
Clouds  constantly  passing,  hiding  42  (c)  Orionis  to  the  naked  eye. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  159 

1879,  FEBRUARY  23. 
Photometer.     (Very  windy.) 

D,  brightest  part:  5.5,  5.0,  4.8;  mean,  5.10  (3) 

E,  brightest  part:  5.1,  53,  5.5;  mean,  5.30  (3) 

E  —  0.98  D 

1879,  MARCH  i. 

8h  40™.     Eye-piece,  400.     Wt.  =  3. 
654  visible;  also  LASSELL'S  b  (double??). 
h  visible;  651  visible;  642?  visible;  not  quite  sure  of  it. 

Schroeter's  second  bridge. 
Visible  as  far  south  as  647  ;  south  of  this  r  is  black. 

No  real  stellar  nucleus  g0,  but  a  condensation  of  nebulous  matter  io"-2o"  large 
is  there,  separated  from  the  rest  of  the  nebula  by  dark  channels  n.  and  s.  of  it.     That 

channel  to  the  south  is  the  blacker. 

D. 

It  is  not  quite  certain  if  the  following  edge  of  D  extends  further  south  than  647  as 
a  continuous  edge.     All  the  attention  J.  could  give  will  not  decide  to-night. 

1879,  MARCH  7. 
Photometer. 

D,  6.4,  5.6,  6.5,  6.9 ;  mean,  6.4  (4) 

This  part  of  D  is  defined  thus:  join  669  and  641 ;  bisect  this  line,  join  its  middle 
point  with  647.     The  mirror  is  10"  north  of  this  line. 

E,  7.1,  7-3,  6-5,  7-o;  mean,  7.0  (4) 

D  (brightest),  6.3,  6.3,  6.1  ;  mean,  6.2  (3) 
A,  4.1  :,  4.2:,  both  doubtful;  mean,  4.2  (2) 

F,  54,  5-9,  54 ;  mean,  5-6  (3) 

In  the  reductions  I  have  used  D0  =  6.3. 

1879,  DECEMBER  26. 

Power,  175.     Wt.  zr  i.     Strong  moonlight,  but  sky  very  clear. 
61 2  and  6 1 8  both  seen ;  h  near  the  point  of  D  seen  like  a  nebulous  mass  and  not 
like  a  star;  a  clear  dark  space  is  between  h  and  the  folktving  side  of  D. 

D. 

The  bright  following  edge  of  D  does  not  (seem  to)  extend  further  south  than  647 
as  very  bright.     South  near  T  it  is  definite  but  fainter. 

Order  of  brightness. 

i.  A,  D.     2.  G,  H,  F,  I,  Q,  E. 

G  and  H  notably  brighter  than  F. 

Moonlight  too  strong  to  examine  the  neighborhood  of  793. 


l6o  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1880,  JANUARY  3. 

Beg-in  ioh,  end  i2h.     Power,  175.     Wt.  —  4,  and  very  clear. 

675  seen  just  outside  of  frons.  LASSELL'S  b  seen.  612  >  618.  618  >  567- 
567  —  636  very  faint.  647  in  a  dark  space.  651  just  on  the  edge  of  D  [with  175; 
with  400  A  it  was  slightly  within  D]. 

h  visible;  nebulous  and  not  stellar. 

ioh  5m,  654  just  barely  visible. 

Neighborhood  of  793. 

The  apex  of  the  curve  of  the  Messierian  branch  near  793  is  probably  n.  p.  that 
star  a  little,  but  the  shapes  of  the  edges  north  and  south  of  this  star  are  such  that  if  a 
drawing  were  made  by  continuing  the  outline  of  the  edge  from  the  south  of  793  north- 
wards, the  Cape  would  be  put  south  of  793,  while,  if  the  drawing  were  begun  north  of 
793  and  continued  toward  the  south  the  Cape  would  be  put  north  of  793.  In  fact 
there  are  really  two  capes  like  this  one  [the  southern  and  fainter  one  ends  in  an 
exceedingly  minute  star,  £,  of  the  drawing  (omitted),  which  is  in  p  —  2oo°-2io°,  s  •=. 
30"  from  793.  This  star  £,  even  if  not  seen,  gives  a  point  for  the  eye  to  rest  on  in 
approaching  793  from  the  south,  and  would  cause  the  apex  of  the  Messierian  branch 
to  be  put  at  t;  approaching  793  from  the  north  the  apex  would  be  put  at  that  bright 
star  (793)]. 

I  am  satisfied  that  the  nature  of  the  object  is  such  that  no  evidence  of  a  change 
can  be  based  on  the  position  of  this  Cape  in  relation  to  793.  [See  observations  of 
1876,  March  14  and  March  22.  These  drawings  (omitted)  confirm  and  agree  with  to- 
night's.] On  an  unpublished  drawing  af  Gr.  P.  BOND'S  I  find  the  statement  that  the 
nebula  reaches  exactly  to  793  (about  March  n,  1861). 

Near  793  are  three  or  four  bright  points  which  could  probably  be  located  by 
measures  if  it  were  worth  while.  They  may  be  very  minute  stars.  One  of  these  is  at 
£,  in  the  figure  (omitted),  i.  e.,  p  =  2oo°-2io°,  s  =  30"  ±  from  793. 

D. 

The  following  edge  of  D  up  to  647  is  brighter  considerably  than  the  following  edge 
of  T  just  south  of  647. 

1880,  JANUARY  3. 

Photometer. 
ioh  50™. 
The  flame  of  the  lamp  does  not  seem  to  burn  satisfactorily. 

E.  4.6 

4.2 
4.1 


4-30  (3) 
I.  Southern  end. 

4-50  (0 
The  above  will  do  for  a  comparison  of  E  and  I. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  161 

Flame  too  low  and  wick  changed  ;  then 

D.  5-0 


4.8 

5-oo  (3) 


Clock  not  running  regularly. 


E.  4.9 

5-o 
4.8 


4-90  (3) 
Measures  of  D  rejected,  not  in  proper  part  of  D 

I.  5-3 
5-6 

5-3 

5-40  (3) 

£.  4.9 

5-o 
4-7 
5-0 

4.90  (4) 

G.  4.7 

4.8 

4-5 
4-67  (3) 

D.  5-' 
5-2 
4-9 

507  (3) 
End  1  2h.  The  clock  stopped  several  times  and  the  measures  not  satisfactory. 

1880,  JANUARY  ic. 

Photometer. 

Images  very  unsteady. 

The  mirror  was  put  in  D  on  the  line  joining  641  and  647,  and  with  its  south  end 
at  647  (its  north  end  was  therefore  about  15"  north  of  647). 
APP.  V  -  21 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Begin  g\ 

D.  6.1  This  is  not  by  any  means 
6.3                   the    brightest    part    of    D ;    the 
6.3                   brightest  of  D  is  farther  east. 

6-23   (3) 

E.  7.3 
7.0 

7-13  (3) 

F.  5-9 


5-53  (3) 

a  4-9 

5-2 

4.8 

4.97  (3) 

F.  5.0 

5-5 
5-3 

5-27  (3) 
D.  4.6 

5-i 

54 
54 

5-i2  (4) 

G.  4-9 
4-7 
5-o 

4.87  (3) 
F.  4.8 

5-2 
5-2 

5-07  (3) 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  163 

E.  6.3 
6.1 

-. 
6.23   (3) 

D.  4.6 

5-2 
5-0 

4-93   (3) 


Summary. 

Do. 

i.  D. 

6.23 

6.  20 

2.    E. 

7-i3 

6.10 

3-  F. 

5-53 

5.80 

4.  a 

4-97 

5-50 

o*      * 

5-27 

5-30 

6.  D. 

5-12 

5.10 

7.  a. 

4.87 

5.10 

8.  F.x 

5-07 

5.00 

9.  E. 

6.23 

5.00 

10.  D. 

4-93 

4.90 

As  the  lamp  was  perpetually  growing  fainter,  I  have  used  interpolated  values  of 
the  readings  on  D,  as  in  the  second  column. 

1880,  JANUARY  10. 

Looked  with  the  finder  and  with  low  power  (175)  for  the  star  LE  GENTIL  puts 
furthest  north  in  his  drawing.  It  does  not  exist.  I  can  only  explain  it  at  all  by  sup- 
posing it  may  be  570  G-.  P.  B..  put  north  of  &  in  d  instead  otsoiith,  but  in  the  same  Ja. 

1880,  JANUARY  13. 

Photometer. 
D  (same  part  as  before  observed). 

7-8 

7-9 
7.6 

7-77  (3) 
E  (bri{/M<*t). 
7-3 

7-3 
7.2 

7-27  (3) 


164  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

I  (brightest). 

7-7 
7.6 

7.0 

7-5 

745   (4) 
F  (brightest).  , 

57 

5-7 
54 

5-6o  (3) 

G-  (brightest). 

6-5 
5-5 
6.0 
6.4 

6.10  (4) 

D.  6.7 

6-3 
6-3 

6.43   (3) 

Then  set  the  lamp  at  7.9,  the  same  reading  as  at  first,  and  found  the  mirror  just 
barely  illuminated ;  so  that  the  lamp  certainly  has  grown  fainter. 

The  above  observations  seem  to  me  very  good,  the  best  this  year,  and  compar- 
able with  those  of  last  winter. 

Summary. 

Dn. 

E.  7.27         7.50 

I.    7.45         7-20 

F.  5.60         6.90 
Gr.    6.10         6.60 

[As  before,  I  have  used  interpolated  values  for  the  readings  on  D,  as  I  experi- 
mentally proved  that  the  lamp  grew  fainter.] 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION,  165 

1880,  JANUARY  14. 

Photometer. 

D;  5.8,  5.6  6.0,  6.1  ;  mean,  5.88  (4) 
E;  5.9,  6.0,  6.4,  6.8,  6.0;  mean,  6.22  (5) 
I;   5.2,  5.4,  4.8;  mean,  5.13  (3) 
D  ;  5-o,  5-9,  5-5»  5-5  5  mean,  5.48  (4) 
F;  4.8,  4.8,  5.0;  mean,  4.87  (3) 
G;  4.6,  4.9,  4.2,  4.1  ;  mean,  4.45  (4) 
E  ;  5A  5.2,  4.8  ;  mean,  5.00  (3) 
D;  5-2,  5-2,  5-i  I  mean,  5.17  (3) 
H;  54<  5-7,  541  mean,  5.50  (3) 

Summary  (using  interpolated  values  of  D). 
D;    5.88,  - 
E;    [5.80],  6.22 
I;    [5.60],  5.13 
D;    5.48,- 
F;    [5.40],  4.87 

Gh  [5-30],  445 
E;    [5.20],  5.00 


H;  [5-20],  5.50 

1880,  JANUARY  16. 

Photometer. 

D;  5-6,  5-8,  55;  mean,  5.6  (3) 
E  ;  5.3,  5-2  ;  mean,  5.3  (2) 
I;   5-o,  5-5,  6.0,  5.4;  mean,  5.5  (4) 
F;  4.8,  5.1,  5.0;  mean,  5.0  (3) 
G;  4.5,  5.0,  5.2;  mean,  4.9  (3) 
D;  5A  5-7,  5-5;  mean,  5.6  (3) 
E  ;  54,  5-5  5  mean,  5.5  (2) 
I;  5-5,  5-o;  mean,  5.3  (2) 
[E]  15"  south  of  the  brightest  parts;  6.4,  6.6,  6.9;  mean,  6.6  (3) 

1880,  JANUARY  21. 

Eye-pieces  400,  600  A. 

D. 

The  nebulous  patch  7?  ,  near  the  n.  f.  point  of  D,  is  2//-3//  on  a  side;  it  has  dark 
spaces  all  round  it.     The  Index-Chart  compared  with  the  sky  as  follows  : 

E. 
The  n.  f.  angle  of  E  in  the  chart  is  about  90°  ;  in  fact,  it  is  less  than  90°. 

F. 

The  11.  point  of  F  is  correctly  drawn  ;  that  is,  it  is  -north  of  the  north  point  of  E 
considerably. 


1 66  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Channel  between  X  and  F. 

This  (although  barely  seen  to-night)  is  not  correct  on  the  chart ;  it  is  not  perpen- 
dicular to  the  channel  separating  E  and  F. 

I. 

The  south  side  of  I  (chart)  is  too  long  compared  to  the  north  side  of  E. 

F,  G,  H. 

The  channel  between  F  and  (G  and  H)  is  right  in  the  chart  at  the  east  and  west 
ends,  but  it  is  rhomboidal  in  shape ;  and  the  width,  as  given  in  the  Index-Chart,  just 
north  of  the  letter  F,  is  not  more  than  one-third  of  the  true  width  there.  In  fact,  the 
whole  mass  H  (in  the  chart)  should  be  moved  north  and  east  a  considerable  distance  as 
my  measures  show.  Otherwise,  Gr  and  H  are  about  right,  i.  e.,  as  to  shape. 

Q. 

The  details  of  Q  cannot  be  made  out  in  this  bright  sky,  but  the  shaded  portions 
of  the  chart  near  stars  676  and  686,  while  they  are  right  enough  in  themselves,  give  a 
wrong  impression.  The  whole  space  directly  south  of  T  should  be  shaded  far  enough 
to  join  with  the  space  ij. 

The  Sinus  Lamontii  is  not  satisfactorily  laid  down. 

A. 

This  mass  is  not  well  drawn,  but  the  figure,  with  the  explanations  as  given  in  my 
various  measures,  will  explain  it.  A  good  figure  of  this  mass  would  be  very  complex 

\Vi  (lacus  Secchii). 

Should  be  more  nearly  circular,  and  its  position  in  the  chart  as  to  dot  and  dS  is 
not  correct. 

D. 
is  tolerable  well  drawn  as  to  shape. 

T. 
should  be  faint. 

W2,  "Wa,  etc.,  cannot  be  well  seen  in  this  bright  sky,  nor  can  any  of  the  fainter 
and  outlying  parts. 

1880,  JANUARY  29. 

Eye-piece,  400  A.     Wt.  —  3. 

612  and  618  visible ;  north  and  east  of  them  is  a  dark  space,  which  is  only  partly 
shown  on  the  Index-Chart. 

W1. 

is  not  well  drawn  as  to  shape,  and  W4  is  correct ;  W2  and  W5  not  well  seen  on  account 
of  moonlight. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  167 

B. 

575  is  the  southern  limit  of  B  [this  is  different  from  Lord  ROSSE'S  drawing]. 

C. 
The  dark  curved  channel  just  north  of  C  is  darker  than  that  south  of  it. 

T. 

The  south  edge  of  T  is  near  the  place  of  654  (not  visible  to-night),  and  it  is 
tolerably  sharply  denned,  more  so  than  is  shown  in  the  Index-Chart ;  it  seems  as  if 
the  Sinus  Lamontii  should  be  continued  past  622,  625,  648,  and  654  in  a  nearly  straight 
line  to  join  with  T  near  [c]. 

T. 

The  south  edge  of  T  from  [c]  to  the  bridge  of  SCHROETER,  is  much  fainter  than  the 
edge  of  T]. 

The  moonlight  is  too  strong  for  the  finer  details. 

SYNOPSIS  or  THE  PRECEDING  DETAILED  OBSERVATIONS. 

In  the  pages  immediately  following  I  have  collected  all  or  nearly  all  the  single 
observations  relating  to  each  particular  bright  mass,  dark  channel,  etc.,  and  have 
arranged  them  chronologically  for  coavenience  of  reference.  The  measures  in  this  sec- 
tion are  corrected  for  differential  refraction.  In  general,  the  essential  portions  of  the 
original  observations  are  alone  given,  but  occasionally  where  the  description  is  too 
long  or  too  unimportant  to  be  repeated,  I  have  added,  after  the  date,  "see  observations." 

By  this  arrangement  it  will  be  easy  for  any  one  to  examine  into  the  accuracy  of 
the  original  observations,  for  under  each  head  each  statement  can  be  examined  in 
detail  and  verified  or  disproved.  The  various  masses,  etc.,  are  here  arranged  nearly 
in  the  order  of  right  ascension.  In  the  following  pages  are  given  the  observations  of 
supposed  variable  stars,  the  examination  of  various  drawings,  the  order  of  brightness  of  the 

various  masses,  etc. 

Frons. 

1875,  Oc.  29.  p  =  ^°-3  (5) 

1875,  Nov.  ii.  p  =  5 2°. i  (4).     This  is  the  best  tangent  to  the  wlwk  line  of  light, 
but  it  cuts  off  some  masses  at  5.  p.  corner  of  E  and  some  at  Q. 

1876,  Nov.  22.  p  —  49°-3  (4) 

1877,  Jan.  24.  The  frons  is  convex  to  the  east,  as  remarked  by  Lord  ROSSE. 

greatest  convexity  is  near  685. 

1877,  Feb.  7.  Convex  as  above. 

1878,  Jan.  1 6.  p  =  5*0-3  (4),*  =  '93".5  (3)-     The  measures  of  *  [length  of fron 
were  made  to  compare  with  LIAPONOFF  ;  but  they  are  very  uncertain  from  the  i 

of  the  case.     HOLDEN-LIAPONOFF  =  +  3".  7. 

1879,  Jan.  1 8.  #  =  49-2  (4)-     Wt=  i. 

Occiput. 
1875,  Oct.  29.  #=142°.!  (5).     Occiput  exactly  parallel  to  line  506-570. 


1 68  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1875,  Nov.  10.  The  line  joining  R  56  and  581  is  perpendicular  to  occiput. 

1875,  Nov.  ii.  p=  139°. 3  (4).     This  measure  cuts  Y  off  entirely. 

1876,  Mar.  6.  p=  142°. 7  (4) 

1876,  Nov.  22.  p  •=.  1 36°. 6  (4).     This  measure  cuts  off  a  little  of  E,  and  is  hardly 
satisfactory. 

1877,  Jan.  5.  p—  135°. 5  (3).     This  measure  cuts  off  Y.     Wt.  —  i. 

N.  B.  The  last  two  observations  are  not  comparable  with  the  preceding-  three. 

1877,  Jan.  5.  s  (from  9')  io6".5  (3);  HOLDEN-LAMONT  =  -f-  io".2;  HOLDEN-LIA- 
PONOFF  —  +  io".i." 

1878,  Jan.  1 6.  |;—  I39°,6  (7).     This  is  the  best  tangent  from  south  point  of  J  to 
south  point  of  E.     It  cuts  off  Y. 

1879,  Jan.  18.  j0—  143.2  (4).     Wt.  —  i.     Preceding  edges  of  E  and  I. 

Sinus  Gentilii  (/?,  K,  and  y). 

1875,  Nov.  10.  If  there  is  any  totally  black  inlet  from  the  south  in  the  Sinus  it  is 
very  narrow. 

1876,  Feb.!  The  Sinus  Gentilii  is  connected  with  V  by  a  channel  running  NE. 
(See  observations.) 

1876,  Feb.  ?  4S  north  end  —  —  66".  7. 

1876,  Mar.  14.  The  line  joining  573  and  575  is  the  best  tangent  to  the  preceding 
shore  of  y. 

1877,  Jan   2.     As  in  Index-Chart.     It  is  pretty  uniformly  black,  but  not  so  black 
as  r',  etc. 

1877,  Feb.  7.  North  point  of  Sinus  Gentilii  Jd  =  -\-66".7  (4) 

1877,  Dec.  12.  The  line  573-575  as  on  1876,  March  14. 

1878,  Mar.  5.  Northendof  y;  Jd  —  —  ^".^  (4)  or— 63". 3  (4).    (See observations.) 


1876,  Feb.?  North  point;  Jd  =  —io2".6. 

1877,  Jan.  2.  As  in  Index-Chart. 

1878,  Mar.  5.  North  end;  Jd  =  —  99".2  (3) 

J. 

1877,  Jan.  2.  See  observations. 

1878,  Mar.  9.  Preceding  point ;  ^#  —  —  97"-3  (4) 

c. 

1875,  Nov.  5.  The  branch  is  about  right  in  the  Index-Chart,  perhaps  a  little  too 
much  curved.     It  runs  a  little  to  the  north  of  523,  but  that  star  is  involved  in  nebu- 
losity to-night. 

1876,  Mar.  6.  About  right  on  Index-Chart. 

1877,  Jan.  2.  c  about  as  bright  as  B  (roughly  speaking). 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  169 

J  AND  B. 

1875,  Oct.  29.  p  —  7°. 7  (3)  angle  of  preceding  edges. 

1876,  Nov.  27.  p  —  5°. i  (i)  angle  of  preceding  edges,  which  pass  through  575  in 
this  measure. 

1876,  Dec.  19.  p=z  1 2°. 2  (3) 

1878,  Jan.  16.  jM-rz  7°. 7  (3)  this  measure  passes  through  575. 

B. 

1875,  Oct.  29.  B  points  exactly  to  575,  which  is  at  the  very  end  of  it.  Position 
angle  of  B  from  its  north  end  to  575  —  6°.o  (i) 

1875,  Nov.  24.  567  half  way  from  B  to  i,  in  a  dark  space. 

1876,  Dec.  19.  B  extends  no  further  than  575  as  a  bright  mass,  certainly  not  as 
much  further  as  is  given  by  Lord  ROSSE,  1867. 

1876,  Dec.  31.  B  runs  exactly  through  575,  and  if  it  extends  south  of  575  as  a 
distinct  mass  it  is  considerably  fainter.     The  branch  (B)  leading  to  575  is  brighter 
than  the  branch  (in  A?)  leading  to  589. 

1877,  Jan.  2.  About  half  way  from  575  to  north  end,  B  is  about  as  bright  as  the 
general  mass  of  I. 

1877,  Dec.  12.  Tangent  to  the  north  side  of  the  curve  in  which  B  joins  A,  46  = 
+  3°"-5  (4)-     B  ig  much  fainter  south, of  the  parallel  of  &  than  north  of  it;  it  seems 
hardly  to  reach  575,  as  a  bright  mass. 

1878,  Jan.  23.  See  observations  on  brightness  of  B. 

1880,  Jan.  29.  575  is  the  southern  limit  of  B.  This  is  very  different  from  Lord 
ROSSE'S  drawing. 

W1,  W2,  W3,  W4,  W5. 

W1  —  lacus  Secchii  (discovered  by  SCHROETER,  1 794). 

1875,  Nov.  10.  The  distance  from  [the  center  of]  W1  to  685  is  about  equal  to  the 
distance  635-669.  W2  remarkably  black. 

1875,  Nov.  17.  Middle  of  W1,  ^«  =  -37":(i) 

1876,  Jan.  30    W1  blacker  than  V. 

1876,  Mar.  14.  Preceding  side  of  W1,  4 a  —  —  57".3  ;  following  side  of  W1,  Jot  — 
—  29".!.  Middle  of  W1,  4a  ==  —  43".2,  according  to  above,  but  the  preceding  end  of 
W1  is  very  narrow,  almost  a  canal. 

1876,  Dec.  19.  W1  brighter  than  Sinus  Gentilii. 

1876,  Dec.  31.  W1  connects  with  dark  channel  north  of  2,  which  runs  towards  524. 
W1  blacker  than  W4,  and  both  blacker  than  W3,  which  is  not  clearly  outlined  to-night. 

1877,  Jan.   10.  W4  connects  with  W2  (verified)  ;  W3  is  just  south  of  635. 
1877,  Nov.  20.  4d  of  middle  —  +  69".8  (4),  not  very  certain. 

1877,  Dec.     7.  48  of  middle  =  +  67/7.7  (3),  much  better  than  observation  of 
November  20. 

1878,  Jan.   23.  Jd  of  middle  —  +  68".o  (7) 
1878,  Mar.    9.  Ja  of  center  —  —  47"-4 :  (2) 
1878,  Dec.    6.  J<x  of  center  =  —  48". 2  (4) 

5 22 


170  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

A. 

1875,  Nov.  ii.  See  observations. 

1876,  Feb.  ?  South  end  of  A  near  star  622,  4d  —  —  3i".8. 
1876,  Mar.    6.  Star  622  from  &  Orionis,  p  —  i99°.o  (i) 

1876,  Dec.  19.  See  observations.     Some  of  the  details  are  different  from  LASSELL, 
1863. 

1876,  Dec.  31.  See  observations.     589  at  south  end  of  a  bright  part  of  A. 

1875,  Oct.  27.  589  exactly  on  preceding  edge  of  A.     This  edge  is  very  faint  com- 
pared to  following  .edge. 

1877,  Dec.    7.  4d  south  point  =  —  3i".9  (2),  not  very  certain. 

1877,  Dec.  12.  Jd  south  point  —  —  3i".2  (5).    See  observations  for  position  of  622. 

I. 

1876,  Mar.  14.  The  rounded  apex  (south  point)  of  I  is  bisected  in  4 a  rr. —  29".!. 

1877,  Jan.  24.  No  nucleus  (602)  seen  to-night.     [It,  however,  exists.    See  1874, 
January  17,  etc.] 

1877,  Dec.  12.  The  s.  p.  corner  of  I  projects  beyond  [preceding]  the  general  line 
of  the  occiput. 

1878,  Jan.  23.  S.f.  corner  of  I ;  JS  —  —  95". 8. 

1878,  Jan.  28.  S.f.  corner  (near  Aj)  ;  JS  —  —  77".4  (i) 

1878,  Mar.  5.  See  observations  for  the  relation  of  I  to  the  parallel  Jd  —  —  77"  ; 
also,  this  date  5.  /  corner ;  Jd  —  —  97". 6  (i)  ;  corner  near  Ax ;  46  =.  —  8i".2. 
1878,  Dec.  6.   West  point  of  I  (near  AJ  ;  Ja  —  —  6 2". 8  (4) 

Dark  channel  between  U  and  A. 

1876,  Dec.  31.  Much  as  in  Index-Chart.     See  observations. 

1877,  Jan.  24.  622  in  the  channel  half  way  from  edge  of  U  to  edge  of  A. 

L. 

1875,  Nov.  ii.  See  observations. 

1876,  Feb.    1  .  North   end  on  same  parallel  as  671,  and   the  east  point  of  Q; 
48  —  —  25".6. 

1877,  Jan-  24-  L  from  621  to  601,  and  from  thence  to  595  (i.  e.,  the  north  shore) 
is  very  bright,  nearly  as  bright  as  A  near  it. 

1877,  Dec.  12.  Jd  north  point  —  —  25".4  (4)  ;  about  the  same  at  671. 
1.877,  Dec.  12.  The  preceding  edge  of  L  near  60 1  prolonged  would  pass  through 
the  s.  p.  corner  of  I. 

*?• 

1875,  Nov.  24.  South  shore  about  right  in  Index-Chart. 

1875,  Dec.  19.  As  in  Index-Chart. 

1875,  Jan.    2.  See  observations. 

1877,  Dec.  12.  The  line  570-666  is  approximately  the  south  border  of  rj. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  171 

Y. 

1877,  Jan.    2.  Y  is  not  a  marked  feature  of  E,  as  in  LASSELL,  1863,  but  requires 
attention  to  see  it. 

1878,  Dec.  ii.  See  observation. 

E. 

1875,  Nov.  10.  South  extremity;  Jd  —  —  I47".6  >    .  _       „ 
1875,  Nov.  10.  North  extremity ;  Jd  rr  —  ioi".7  ) 

1875,  Nov.  17.  Apex  of  E  almost  exactly  south  of  &  ;  i.  e.,  Ja  —  o".o. 

1876,  Feb.  1  South  point  of  E,  Jd  —  —  142"^,  145".!,  i44".2;  weaw,  —  I44".o. 
1876,  Feb.  I  -ZVortf/fc  point  of  E,  z/£  —  —  io2".6  (approximately). 

1876,  Mar.  14.  Apex  of  E,  Ja  =  —  $".$  (poor  images). 

1877,  Jan.  2.  E  nebulous  [no  nuclei]  and  uniform  in  brilliancy. 

1877,  Feb.  6.  The  apex  of  E  certainly  does  not  precede  0'.  It  appears  to  be  in 
same  R.  A.  (9h.o).  It  may  follow  it  2"-$". 

1877,  Feb.  7.  The  following  side  of  E  is  nearly  a  straight  line. 

1877,  Dec.  7.  4d  of  south  point  —  i43"-5  (4);  the  smallest  J6  which  smaller 
telescopes  could  give  is  not  less  than  i3i//4  (i) 

1877,  Dec.  12.  Jd  of  south  point  —  155".  6  (2)  [extreme]! 

1878,  Jan.  28.  Jd  of  south  point  —  i47//-2  (3) 

1878,  Feb.  28.  The  brightest  part  is  the  center  of  the  mass. 
1878,  Dec.  n.  E  uniform  in  brightness  all  over;  the  n.  f.  corner  appears,  pos- 
sibly, a  little  brighter  ! 

1880,  Jan.  21.  The  n.f.  angle  of  E  is  less  than  90°. 

Z. 

1878,  DECEMBER  n  (see  observations). 
Dark  channel  between  I  and  (X  and  Gf). 

1876,  Mar.  6.  A  straight  portion  of  some  length,  whose  direction  prolonged  passes 
through  570,  p  =1  25°.6  (3). 

Dark  channel  between  I  and  E. 

1876,  Dec.  5.  Its  prolongation  precedes  671.    p  =  42°.6  (4)  ;  quite  uncertain. 

1876,  Dec.  31.  It  is  about  parallel  to  frons  [p  =  50°  ?]. 

1877,  Dec.  12.  Its  north  border  prolonged  is  very  approximately  tangent  to  the 

n.  p.  end  of  F ;  i.  e.,  X. 

1878,  Jan.  1 6.  jp  =  46°  (i);  uncertain. 

Dark  channel  between  E  and  F. 

1875,  Nov.  24.  j?rri39°.6  (2).     Its  direction  passes  through  the  star  589.     For 
further  description,  see  observations. 
1878,  Jan.  1 6.  p=  i4l0-7  (3) 


172  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Dark  channel  between  F  and  (G  and  H). 

1875,  Nov.  17.  See  under  F,  synopsis. 

1876,  Mar.  6.  Its  prolongation  is  in  the  line  685-708. 
1876,  Dec.  19.  Same  as  March  6. 

1876,  Dec.  31.  Same  as  March  6. 

1877,  Jan.  5.  Same  as  March  6. 
1877,  Jan.  24.  Same  as  March  6. 

V. 

1875,  Nov.  5.  The  ground  on  which  the  trapezium  stands  is  not  totally  black. 

1875,  Nov.  17.  V  not  so  black  as  r' . 

1876,  Jan.  30.  V  not  so  black  as  Wl 

1876,  Mar.  14.  V  not  so  black  as  Sinus  Gentilii,  but  comparable  with  and  almost 
as  black  as  north  half  of  r'. 

1877,  Jan.  24.  618  inside  V;  61*2  in  nebulosity  or  very  close  to  border. 
The  extent  of  V  towards  the  north  is  therefore  (612)  ;  46  —  -f  24".o. 

1878,  Mar.  5.  South  point;  Jd  —  —  22".^  (3) 

F. 

1875,  Nov.  10.  Brightest  part,  ^£  —  —  ioi".o  (  —  LIAPONOFF'S  a0)  ;  his  measures 

give  97".4- 

1875,  Nov.  17.  The  south  part  of  F  almost  stellar  in  appearance. 

1875,  Nov.  24.  The  line  through  685  and  708  passes  through  the  brightest  part 
of  F  (i.  e.,  X  [?]),  and  is  parallel  to  the  black  channel  between  F  and  (G  and  H). 

1876,  Feb.?  South  point,  JS-—  122".$. 

1876,  Mar.  14.  Tangent  to  preceding  sides  of  F  and  G,  da  —  -f-  \" .2. 
Mem.  X  must  be  too  far  east  in  Index-Chart. 

1876,  Mar.  14.  F  is  bisected  very  nearly  in  Ja  —  -f- i8".8  (28//.8). 

1877,  Feb.  3.  Intersection  s.  p.  edge  with  frons,  da—  19".  2  :  (3) 
1877,  Feb.  7.  (Brightest  part  which  is  not  X),  Jd  =  —  102". i  (4) 
1877,  Jan.  2.  Nucleus  stellar  to-night. 

1877,  Jan.  24.  X  and  F  seem  to  be  almost  separated  by  a  fainter  streak  nearly  in 
the  parallel ;  following  end  of  F  fainter  than  the  rest  of  the  mass. 

1877,  Dec.  12.  South  point  of  Fin  frons,  JS  =  —  ii7".3  (2) 
Extreme  north  point  (X),  Jd  —  —  94". 9  (2) 

1878,  Jan.  28.  Brightest  part,  JS  —  —  102". 9  (4)  ;  da  —  +  26".8  (2).     G  >  F. 
1 878,  Mar.  5.  J6  south  point  preceding  the  frons,  —  1 1 3". 5  (3).    See  observations. 
1878,  Mar.  5.  A  faint  channel  visible  through  the  following  end  of  F. 

1878,  Mar.  5.  Brightest  part,  Jd  —  —  101" .2  (2) 

1878,  Dec.  ii.  F,  G,  H  about  equal  in  brightness,  and  perhaps  F  is  the  brightest. 
This  is  doubtful.     The  brightest  part  of  F  is  south  of  X. 

1879,  Jan.  20.  Brightest  part  of  F,  J6-—  102". 8  (4) 

1880,  Jan.  21.  The  north  point  of  F  is  north  of  the  north  point  of  E. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  173 

G. 

1875,  Nov.  10.  Brightest  part,  Jd  —  —  64".$  (—  LIAPONOFF'S  60);  he  gives  —  63".$. 

1875,  Nov.  17.  The  middle  part  almost  stellar. 

1876,  Feb.!  Center  and  brightest  part,  J6-  —  6g".6. 

1876,  Mar.  14.  Tangent  to  preceding  edges  of  F  and  G,  Ja  —  -f  \" ,2. 
1876,  Mar.  14.  Tangent  to  following  side  of  G,  J6  —  -f-  28".8. 

1876,  Dec.  5.  4d  (brightest  part),  —  65".5  (5)  ;  poor  measures. 

1877,  Jan.  2.  Nucleus  stellar. 
1877,  Jan.  24.  See  observations. 

1877,  Feb.  3.  G  (middle  point),  Ja  —  -f-  i^'.g  :  (3) 

1877,  Feb.  6.  The  preceding  side  of  G  comes  nearly  to  the  R  A.  of  & '. 

1877,  Feb.  7.  G  (middle  point),  Jd  —  —  66". i  (4) 

1877,  Nov.  20.  4d  (brightest  part),  =  —  6o".9  (3).     Images  very  poor. 

1878,  Jan.  28.  46  (brightest  part),  —  —  66".5  (3)  ;  Jd  (south  point)  —  —  75".  7  (4) 
1878,  Jan.  28.  Ja  (brightest  part),  —  -f-  i$".g  (3) 

1878,  Mar.  5.  All  of  G  is  north  of  --  77". 

1879,  Jan.  20.  Brightest  part  of  G,  Jd  —  —  64.". 9  (4) 

H. 

1877,  Dec.  12.  JS  south  end  of  JI  in  frons=.  —  94". 21  (i) 

1878,  Mar.  5.  Jd  south  edge  =  —  77".5  (3).     [This  is,  of  course,  within  the  line 
of  fronsJ] 

1878,  Dec.  ii.  The  axis  of  H  is  nearly  in  the  line  685-622.     The  line  of  the 
following  edges  of  H  and  M  (prolonged)  passes  between  640-624. 

M. 

1878,  Mar.  5.  North  edge  of  M,  Jd  —  —  22". 7  (3) 

D. 

1875,  Oct.  27.  It  is  suddenly  much  brighter  just  north  of  647. 

1875,  Nov.  10.  Bright,  sharp  n.  f.  end,  46  =  -f  77"-7  (3) 

1876,  Mar.  14.  Tangent  to  brightest  and  folloiving  side,  Ja  =  +  28".8. 

1876,  Dec.  19.  651  is  precisely  on  the  edge  of  D;  i.  e.,  Ja  =  +  28".8. 

1877,  Jan.  12.  The  brightest  part  follows  the  line  635-647  a  little. 
1877,  Feb.  3.  Tangent  to  folloiving  edge,  Ja=3o".$  (3) 

1877,  Nov.  20.  [JS  —  Si".  :  :]     See  observations. 

1877!  Dec.  12.  Jd  of  north  point  not  seen  well  enough  to  measure. 
1878'  Jan.  1 6.  The  north  point  is  not  sharp  nor  bright  to-night. 

1878,  Jan.  28.  Ditto.     See  observations  in  detail. 
1878,  Jan.  28.  Ja  (following  edge)  =  -j-  29^.6  (3) 

1878,  Dec.  6.  4a  (following  edge)  z=+  29/7.3  (4) 

1879,  Mar.  i.  See  observations. 

1878,  Feb.  28.  The  brightest  part  is  on  the  line  647-663. 

1878*  Mar.  9.  647  is  on  the  following  and  north  side  of  a  channel. 


I  74  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1879,  Dec.  26.  The  bright  following  edge  of  D  does  not  (seem  to)  extend  farther 
south  than  647  as  very  bright;  south  near  T  it  is  definite  but  fainter.     See  observa- 
tions, 1880,  Jan.  3. 

1880,  Jan.  3.  651  just  on  the  edge  of  D  (with  175)  with  400  A  it  was  slightly 
within  D. 

1880,  Jan.  21.  The  nebulous  patch  h  is  2"-$"  on  a  side. 

Dark  channel  following  D  and  preceding  v. 

1875,  Nov.  24.  .It  certainly  does  connect  with  r. 

r. 

1875,  Nov.  5.  South  third  is  darker  than  north  two-thirds. 
1875,  .Nov.  10.  North  half  is  filled  with  light  which  joins  on  to  pons  Schroeteri. 
1875,  Nov.  1 1.  North  third  is  too  bright  in  G.  P.  BOND'S  drawing. 
1875,  Nov.  17.  South  third  is  blacker  than  north  two-thirds. 

1875,  Nov.  24.  The  preceding  and  south  edges  of  r  are  bordered  by  a  very  black 
stripe.     (Contrast!1?)     The  south  third  is  black,  the  north  two-thirds  full  of  nebulosity. 

1876,  Jan.  3.  The  north  half  is  filled  with  faint  nebulosity;  the  south  half  is  empty. 
Half  way  between  the  following  edges  of  D  and  the  preceding  edge  of  pons  Schroeteri, 
there  is  certainly  a  faint  bright  bridge  of  light  as  sketched  to-night  on  the  Index-Chart, 
similar  to  SCHROETER'S  second  bridge.     Its  base  is  connected  on  the  north  to  v,  and  it 
extends  towards  the  south  as  far  as  the  parallel  of  the  bright  nucleus  of  pons  Schroeteri, 
[46  —  39"  approximate]. 

1876,  Jan.  3.  The  preceding  side  of  r  is  blacker;  i.  e.,  there  is  a  black  stripe  bor- 
dering D  and  T. 

1876,  Jan.  3.  South  half  of  T  brighter  than  usual.     See  observations. 

1876,  Jan.  4.  The  east  side  of  r  not  so  black  as  the  west;  the  north  part  not  so 
black  as  the  south. 

1876,  Jan.  30.  Quite  black  on  preceding  side;  a  thin  black  streak  edges  all  the 
west  and  north  sides  of  T  ;  the  north  half  of  r  is  decidedly  nebulous,  while  the  south 
half  is  almost  jet  black. 

1876,  Feb.?  All  north  of  ^tfzz-f  47".!  in  r  is  filled  with  nebulosity,  except,  of 
course,  the  black  channel  on  its  west  edge ;  nearly  all  south  of  this  is  pretty  black, 
though  not  so  black  as  r'. 

1876,  April  i.  North  half  much  brighter  than  south  half. 

1876,  Dec.  19.  South  half  very  black.     651  is  precisely  on  the  edge  of  r. 

Schroeter's  second  bridge  (in  T). 

1876,  Jan.  3.  See  under  T. 

1876,  Nov.  5.  It  is  much  as  previously  drawn. 

1876,  Dec.  19.  It  extends  to  the  south  as  far  as  the  parallel  of  647  [i.  e.,  46-=. 
+  38".o. 
G.  P.  B.].     Its  position  angle  is  a  little  greater  than  that  of  pons  Schroeteri. 

1876,  Dec.  31.  Does  not  extend  south  of  gQ. 

1877,  Jan.  2.  Only   north  two-thirds  seen.     Its  general   appearance   like  Lord 
ROSSE'S  figure. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  175 

1877,  Feb.  7.  It  is  best  defined  on  the  following  side. 

1878,  Dec.  6.  See  observations. 

1879,  Mar.  i.  Visible  as  far  south  as  parallel  of  647 ;  south  of  this  r  is  black. 

Pons  Schroeteri  (00). 

1875,  Nov.  5.  The  nucleus  does  not  seem  to  be  stellar. 

1875,  Nov.  10.  Brightest  part,  Jd=-\-^9".^  (5)  on  same  parallel  as  647  '[+38".o, 
G.  P.  B.]. 

1875,  Nov.  ii.  The  micrometer  wire  through  the  pons  passes  through  685  and 
669  nearly.  p  =  170°. 7  (3) 

1875,  Nov.  17.  The  nucleus  is  almost  stellar.  It  is  very  bright,  almost  as  much 
so  as  the  south  edge  of  ff. 

1875,  Nov.  17.  Brightest  part,  Ja  —  +  79".$  (4).     The  north  part  precedes  this  a 
little,  ^—  175°  about  (estimated). 

1876,  Jan  3.  Nucleus  stellar  beyond  a  doubt,  not  the  whole  nucleus  but  a  cen- 
tral point. 

1876,  Jan.  30.  Center  distinctly  concentrated,  almost  stellar. 

1876,  Feb.  1  Middle  of  the  break,  in  the  pons  Schroeteri,  which  is  south  of  the 
nucleus,  46-=:  -f  22".o  (i) 

1876,  FebJ  Brightest  part,  JS  —  -f  4i".9. 

1876,  Feb.!  Middle  of  the  break  in  the  pons  Schroeteri,  which  is  north  of  the 
nucleus,  Jd  —  +  6o".o  (i) 

1876,  Mar.  14.  Nucleus,  Ja  =  +  76". 4. 

1876,  Dec.  19.  p  —  171°. 2  (3) 

1877,  Jan.  5.  p  =  165°. 8  (2).     Wt.  —  i-     This  is  rather  to  be  considered  as  the  p 
of  a  line  joining  669  and  00,  and  is  a  poor  measure. 

1877,  Jan.  10.  j?rr  175.8  (3).     Wt.  rr  i.     Images  unsteady  but  brilliant. 

1876,  Dec.  19.  0o  is  in  a  central  nucleus  surrounded  by  an  annul  us. 

1877,  Feb.  3.  00,  ^«=77"-5  :  (3) 

1877,  Feb.  7.  g0  not  stellar  in  appearance. 

1877,  Nov.  20.  0o,  J6  =  -{-4i".6  (2).     Images  poor. 

1877,  Dec.  12.  0o,  ^<J  =  +  4o".3  (4) 

(  1878,  Jan. 28.  Ja=-t-76".6  (3);  no  nucleus. 
i8;8,Jan.i6.  jp=i67°.5(3)  uncertain  |  ^  Dec  6    ^a=+76".7(4) 

1879,  Mar-  *•  For  description  of  00>  see  observations.     Not  stellar. 

ff. 

1875,  Nov.  5.  Along  the  south  edge  it  is  notably  brighter. 

1875,  Nov.  5.  Much  brighter  on  south  edge,  then  fainter.  Roughly  speaking,  the 
preceding  half  is  brighter  than  the  following  half;  the  north  shore  is  concur,  rewards 

the  south?? 

1875,  Nov.  10.  8.  f.  sharp  point  of  ff  =  Spitze ;  4d  :      f-  78'  .4  (4) 
1875,  Nov.  1 1.  P  —  89°-7  (4)  ;  general  trend  of  south  shore. 


i;6  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1875,  Nov.  ii.  In  G.  P.  BOND'S  drawing  the  Spitze  is  relatively  too  bright,  but 
not  much. 

1875,  Nov.  17.  The  south  edge  of  ff  for  the  east  two-thirds  of  its  length  is  notice- 
ably and  suddenly  brighter  than  the  rest  of  G. 

1875,  Nov.  17.  Extreme  following  end  of  G-  4a=.-\-  169". 9  (3). 

1876,  Jan.  30.  South  edge  terminates  in  a  sharp  and  bright  edge ;  i.  e.,  compara- 
tively mnch  brighter  than  r,  for  example. 

1876,  Feb.?  South  edge  of  G;  Jd =  -f-  83".  7  near  pons  Schroeteri  f  (approximately). 

1877,  Jan.  30.*  P  of  south  shore  (through  tourmalines  to  compare  with  LE  GENTIL, 
etc.);  p  =  44°  (i) 

1877,  Feb.  3.  4 a.  following  point  of  G  —  -j-  165".!  (3)  ;  good. 

1877,  Feb.  7.  South  edge  of  following  point;  Jd  =.  75"-7  (4)-  From  lacus  Las- 
sellii  east  to  Spitze  the  south  shore  of  o  is  concave  to  the  south. 

1877,  Dec.  12.  South  shore  concave  toward  south  ;  Jd  of  Spitze  79".6  (4)  ? 

G  and  Spitze. 

1878,  Jan.  23.  The  sow^  folloiving  point  of  tf  is  about  on  same  parallel  with  n.  p. 
point  of  D,  and  with  center  of  W1 ;  i.  e.,  Jd  —  -f-  68"  approximately. 

1878,  Jan.  28.  South  shore  of  Spitze;  ^d  —  -\-  8o".7  (4)  ;  4a  —  +  i62"-8  (3) 
1878,  Dec.  6.  ^<*  —  +  i6i".3  (4) 

Lacus  Lassellii. 

This  is  laid  down  in  SCHROETER'S  map  of  1 794. 
1875,  Nov.  5.  652,  657,  663  precede  its  axial  line. 

1875,  Nov.  17.  See  observations. 

1876,  Dec.  19.  It  connects  with  Sinus  magnus. 

Tr. 

1875,  Nov.  5.  On  the  folloiving  side  of  pons  Schroeteri  it  is  quite  dark;  the  line  in 
Index-Chart  seems  to  limit  this  dark  space  properly. 

1875,  Nov.  ii.  G.  P.  BOND'S  drawing  gives  its  north  third  too  bright  relatively. 
•It  is  now  quite  dark  there. 

1875,  Nov.  17.  T'  blacker  than  any  neighboring  part.  North  of  on  prolonged 
(towards  the  west}  it  is  brighter,  but  still  very  faint.  T'  blacker  than  V. 

1875,  Nov.  24.  Just  following  pons  Schroeteri  it  is  very  black. 

P,  S,  M. 

1875,  Nov.  n.  See  observations. 

1878,  Dec.  ii.  676  is  in  the  dark  channel,  but  very  close  to  the  preceding  edge 
of  P. 

Dark  channels  preceding  and  following  M. 

1876,  Dec.  31.  See  observations. 

1877,  Jan.     2.  See  observations. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  !  77 

N. 

1876,  Nov.  5.  Two  nuclei  seen  in  N  (see  observations  for  details). 
1878,  Jan.  28.  South  point  (in  /rows)  /  Jd  —  —  ^".2  (2) 
1878,  Mar.    5.  South  point  ;  J6  —  —  75".o  (i) 

Q,  P,  R. 

1875,  Nov.  1  1.  Angle  of  position  of  n.  f.  sides  of  Q,  P,  and  R.     The  line  passes 
through  654  and  624,  or  nearly  so.     This  cuts  off  some  of  R  and  most  of  T     p  = 
io5°.8  (4) 

1876,  Nov.  22.  p—  i05°.6  (4).     This  cuts  off  some  of  R. 

Q. 

1875,  Nov.  17.  Following  end  of  Q  in  the  same  R.  A.  as  708  [+  150".  6,  G.  P. 
BOND]. 

1876,  Feb.?  East  point  of  Q,  Jd  —  —  25"  .6. 

1  876,  Nov.  5.  North  side  of  Q  quite  bright  and  sharp;  688  is  on  the  north  edge  of  Q. 

.1877,  Feb.  3.  Following  end  of  Q  a  little  preceding  708,  ^/a—  +  147".  3  (3) 
Wt.  z=  2.  The  extreme  point  is  undefined,  and  these  measures  place  the  point  rather 
too  far  west  than  too  far  east. 

1877,  Feb.  7.  Following  point,  ^£  —  —  31  ".6  (4) 

1877,  Nov.  20.  Folloiving  point,  Jd  r=  —  25/7.7  (3).  This  line  bisects  the  fottoicing 
point. 

1877,  Dec.  12.  Following  point,  .^#  —  —  29".  9  (3) 

1878,  Jan.  28.  Following  point,  Ja=.  -f-  152".  6  (4) 

(  north  end  of  n,  Jd  —  —  35".  5. 
1878,  Mar.  15.  Nuclei  in  Q,  <     .-.it      f        >*  „ 

1   (  middle  of  g,  ^<S  —  —  23  .4. 


0  and  P,  and  the  dark  channel  between  them. 

1876,  Nov.  5.  South  of  the  line  671-676  it  is  black,  and  676  seems  to  be  on  the 
preceding  edge  of  P  or  0. 

r". 

1875,  Nov.  5.  North  of  OTT  it  is  quite  dark;  to  the  south  of  on  it  is  filled  with 
nebulosity. 

1875,  Nov.  17.  North  of  OTT  it  is  quite  dark  close  up  to  the  south  edge  of  ff. 

1875,  Nov.  24.  North  of  OTT  it  is  black. 

1876,  Jan.  30.  Quite  black  north  of  OTT. 

OTT. 

1875,  Nov.  17.  OTT  both  seen  ;  they  are  elongated. 

1875,  Nov.  24.   7T  is  brighter  than  o;   o  is  brighter  than  the  middle  of  OTT. 

1876,  Jan.  30.   o  and  TT  like  nuclei  verging  towards  a  stellar  appearance,  but  not 
so  much  so  as  center  of  pons  ScJtroeteri. 

App.  V  -  23 


178  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1876;  Feb.  I  The  parallel  Jd  —  -(-  60"  passes  about  through  the  center  line  of  the 
west  half  of  ox. 

Sinus  magnus. 

1876,  Nov.  ii.  Best  tangent  to  south  shore;  ^?rr  105°. 6  (4).     See  observations. 

1877,  Jan.  27.  Measure  of  position  angle  of  the  north  shore  of  Sinus  magnus 
through  tourmalines.     See  observations;  ^  =  44°  (i) 

£. 

• 

1875,  Nov.  5.  Filled  with  nebulosity.  There  is  at  least  one  dark  streak  in  £ 
parallel  to  ox  and  just  south  of  it.  [Contrast?] 

1875,  Nov.  24.  Just  south  of  OTT  is  a  narrow  black  channel.  After  this  channel 
is  crossed  (going  southwards)  the  Sinus  is  filled  with  nebulosity  up  to  its  south  border. 

1877,  Jan.  2.  Uniformly  filled  with  faint  nebulosity. 

P- 

1875,  Nov.  24.  Following  ff  there  is  a  curious  repetition  of  the  prow-like  shape  of 
the  Spitze  in  ff ;  it  is  much  fainter  and  is  close  to  0,  so  that  it  looks  like  the  shadow  of 
ff  a  little  distorted. 

1877,  Jan-  2-  (See  observations.) 

Cometic  tails  to  stars  685,  708,  741. 

1875,  Nov.  5.  I  do  not  think  such  exist  as  figured  by  Gr.  P.  BOND.  It  is  darker 
between  685-708  than  between  708-741. 

1875,  Nov.  10.  No  wisps  to  these  stars  seen. 

1875,  Nov.  ii.  (See  observations.)  There  is  certainly  no  such  wisp  or  tail  to 
741  as  given  by  G.  P.  BOND. 

1875,  Nov.  17.  741  (same  as 'Nov.  n)  708,  641  (same  as  Nov.  n). 

1875,  Nov.  24.  As  above. 

1876,  Feb.?  No  tails  to  these  stars.     Mag.  power,  175.     Seeing  poor. 

1876,  Dec.  13.  The  space  between  685  and  708  is  blacker  than  that  between 
708  and  741.  This  is  what  gives  the  appearance  of  veritable  cometic  tails,  which 
never  appeared  to  me  as  they  are  figured  by  BOND. 

Remarks  upon  the  general  form  of  the  Huyghenion  region. 

See  observations,  as  follows:  1876,  Jan.  10;  Dec.  19.  1877,  Jan.  2;  Jan.  5; 
Jan.  10;  Jan.  24;  Jan.  30;  Feb.  3.  1878,  Jan.  23. 

Comparison  of  drawings  directly  with  the  nebula. 

G.  P.  BOND.     See  observations  of  1875,  Nov.  n  ;  Nov.  17.     1876,  Feb.?,  etc. 
LAMONT.     See  observations  of  1877,  Jan.  5,  etc. 
ROSSE.     See  observations  of  1877,  Jan.  10,  etc. 

WINLOCK  and  TKOUVELOT.     See  observations  of  1875,  Nov.  24.     1876,  Jan.  10,  etc. 
HUYGHENS.     See  observations  of  1877,  Jan-  3  and  5- 

Comparison  of  Index-Chart,  as  here  published,  with  the  sky.  1880,  Jan.  21, 
Jan.  29. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  i  79 

COLLECTION    OF    THE    SEPARATE    RESULTS    OF    OBSERVATIONS    OF    STARS. 

Stars  within  the  trapezium. 

It  may  be  once  for  all  said  that  never,  under  the  most  favorable  circumstances, 
were  any  stars  or  points  of  light  seen  or  suspected  within  the  trapezium.  These  have 
been  looked  for  on  various  occasions,  by  myself  and  by  others,  and  in  most  of  the 
cases  when  they  have  been  especially  looked  for,  the  fact  is  noted  in  the  observations 
given  in  chronological  order,  but  I  do  not  think  it  worth  while  to  bring  together  in 
this  place  the  particulars  of  such  searches,  since  they  would  be  at  the  most  but  a  mere 
catalogue  of  dates. 

Stars  in  and  near  the  trapezium. 

The  best  summary  which  can  be  given  of  the  evidence  in  regard  to  the  stars 
alleged  to  have  been  seen  in  and  near  the  trapezium  by  DE  Vico  and  others  is  given 
by  OTTO  v.  STRUVE  (p.  99  of  his  Memoir). 

a  Quant  aux  e'toiles  que  DUMOUCHEL  et  DE  Vico  ont  cru  voir  en  dedans  ou  tout 
pres  du  trapeze  je  dois  dire  que  malgr^  tous  les  soins,  je  ne  les  ai  jamais  aper9us. 
Ayant  eu  des  preuves  de  variability  dans  1'^clat  des  e'toiles  situe'es  pres  du  trapeze,  la 
supposition  devait  se  presenter  a  moi  que  ces  e'toiles  avaient  e'galement  changd  de 
lumiere  dans  1'intervalle  entre  les  observations  romaines  et  les  miennes.  Mais  cette 
supposition  perd  tout  son  poids  si  Ton  considere  que  les  astronomes  de  Rome  n'ont  pas 
reconnu  la  sixieme  e'toile  ddcouverte  par  HERSCHEL  et  que,  d'un  autre  cote*  ni  Sir 
JOHN  HERSCHEL  au  Cap,  ni  M.  LAMONT  k  Mimic,  quoique  leurs  observations  portent  a 
peu  pres  la  meme  date  que  celles  de  DUMOUCHEL  et  DE  Vico,  n'ont  rdussi  a  voir  les 
dites  e'toiles.  Tout  cela  porte  done  a  croire  que  dans  ces  cas,  les  astronomes  remains 
ont  e^e*  sujets  a  des  deceptions  optiques.  La  meme  remarque  s'applique  egalement  a 
I'e'toile  qu'a  Paris  en  1857,  M.  PORRO  a  cru  voir  k  1'mte'rieur  du  trapeze,  observation 
qui  en  apparence  a  trouv^  une  confirmation  dans  une  remarque  faite  k  peu  pres  a  la 
mgme  epoque  a  Rome  par  M.  SECCHI.  Au  moins  je  dois  dire  qu'ayant  examine* 
soigneusement  1'inteYieur  du  trapeze  par  chaque  nuit  favorable  du  printemps  1857,  et 
plus  tard,  je  n'ai  jamais  ape^u  une  trace  de  I'e'toile  en  question,  tandis  qu'en  meme 
temps  1'etoile  de  HERSCHEL,  qui  n'a  pas  M  vue  par  M.  PORRO,  ne  m'offrait  aucune 
difficult^."'  There  have  been  many  stars  reported  in  this  space,  but  the  various  cases 
are  not  worth  reporting ;  but  the  careful  examinations  which  STRUVE,  BOND,  and  others 
have  given  show  these  to  have  been  delusions. 

We  find  in  Mon.  Not.  R.  A.  S.,  xxii,  p.  164,  a  portion  of  a  letter  of  LASSELLS, 
dated  Malta,  1862,  January  30,  in  which  he  announces  the  discovery  of  a  new  star 
within  the  trapezium  of  Orion.  "  It  is  situated  near  theta,  the  principal  star ;  appears 
to  be  about  a  full  magnitude  less  than  that  known  as  •  the  sixth  star,'  and  is  about  one- 
sixteenth*  of  its  distance  from  theta.  Its  angle  of  position  from  theta  i 

*  This  was  afterwards  corrected  to  read  six-tenths  (Mon.  Not.  R.  A.  S.,  xxii,  p.  277). 


180  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

less  than  that  of  'the  sixth  star,'  and  consequently  points  a  little  eastward  of  the  star  at 
the  opposite  angle  of  the  trapezium.     For  verification  I  annex  a  diagram : 


"I  suspect  that  the  position-angle  of  'the  sixth  star'  has  considerably  increased 
since  1852."  This  star  has  not  been  seen  here. 

OTTO  v.  STRUVE'S  variable  stars. 

These  are  distinguished  on  the  Index-Chart  by  being  surrounded  with  a  small 
circle,  and  attention  was  paid  to  their  magnitudes  as  compared  to  the  magnitudes  of 
other  stars  in  the  neighborhood  not  so  suspected  of  variability.  Still,  it  will  be  found 
that  the  materials  for  a  discussion  of  the  relative  magnitudes  of  these  stars  are  very 
poor,  poorer  than  I  should  have  supposed  before  bringing  them  together.  Their 
observation  was  considered  of  secondary  importance,  as  it  was  soon  found  (as  early 
as  1874)  that  their  accurate  observation  would  require  an  amount  of  attention  quite 
inconsistent  with  the  main  object  in  view,  which  was  to  accurately  describe  (and,  if 
possible),  to  picture  the  nebulous  masses  themselves  in  their  relation  to  each  other.  It 
must  be  remembered,  too,  that  atmospheric  conditions  have  much  to  do  with  the 
brightness  of  small  stars,  and  that  it  is  easy  to  become  convinced  of  variations  in  mag- 
nitude of  small  stars  from  their  variation  in  brightness  from  night  to  night,  whereas  in 
many  cases  such  a  change  is  due  largely,  if  not  entirely,  to  changes  in  the  transparency 
of  the  air. 

One  exception  to  my  usual  practice  of  never  allowing  the  observations  on  the 
nebula  to  suffer  on  account  of  time  spent  upon  its  contained  stars,  must  be  noted  in 
the  case  of  h  78  —  654.  This  was  usually  looked  for;  if  it  was  not  at  once  visible, 
ordinarily  this  fact  was  noted;  if  it  became  visible  during  the  work,  this  was  also  noted 
with  the  time,  and  if  it  was  visible  a  comparison  of  it  with  other  stars  was  made. 

With  this  preface  I  collect  below  all  the  observations  on  the  brightness  of  the 
small  stars  of  the  Huyghenian  region* 

*  It  may  be  worth  while  to  record  the  following  star-magnitudes  according  to  W.  HERSCHEL'S  MS.  observations 
(unpublished)  : 

W.  HERSCHEL'S  mags.  G.  P.  BOND'S  mags. 

i776.Nov.li.     1.685  =  628    ......      J628  =  8?3 

2.  708  =  741     ......  708=9.6;    74I  =  IO.O 

3.  640,  =  619  <  708  .      .      .      .         640=  ?;     708=    9.6 

4.  669  very  small       .     .     .     .      jj^lljg 

5.  624  smaller  still. 

1778,  Jan.  26.  640  a  little  larger  than  619. 
1810,  Dec.  31.  734  a  little  larger  than  741     .      . 


_ 

Sir  JOHN  HERSCHEL  gives  some  evidence  of  the  variability  of  the  fifth  star  of  the  trapezium  in  Mem.  R.  A%  S., 
vol.  iii,  p.  187,  et  seq. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

OBSERVATIONS  OF  THE  VARIABLE  STARS. 

1874,  Jan.  14.  No  stars  inside  trapezium. 

1874,  Jan.  1 6.  (641)  —  i6m  ARG.  ;  675  not  seen ;  654  seen  7h  to  7h  2om. 

1874,  Jan.  17.  647  and  651  >  (575)  >  (602)  >  (567)  and  (642)  not  seen. 

1874,  Jan.  17.  675  not  seen. 

1874,  Jan.  23.  (641)  seen;  clouds.     654  suspected.     (Cloudy.) 

1874,  Jan.  24.  No  stars  inside  trapezium. 

1874,  Jan.  25.  (642)  not  seen;  654  not  seen;  675  not  seen. 

1874,  Feb.  5.  (641)  seen.     "Is  709  variable?  it  is  quite  faint." 

1874,  Feb.  5.  675  seen. 

1874,  Feb.  14.  LASSELL'S  b  not  seen. 

1875,  Oct.  27.  666  and  667  just  seen. 

1875,  Oct.  27.  647  >  (641),  647  -  (671),  647  >  (575). 

1875,  Oct.  29.  647  =  (575)  >  (671),  but  the  inequality  is  not  great. 

1875,  Oct.  29.  (575)  >  (573),  651  =(654),  (641)  not  seen. 

1875,  Nov.  5.  647  >  (575)  -  (671). 

1875,  Nov.  ii.  (676)  —(654),  (654)  >  (641),  but  not  much. 

1875,  Nov.  ii.  (654)  >  612  or  618.     LASSELL'S  b  not  seen. 

1875,  Nov.  Ir-  (654)  =622  >  $3 K     Query  62 1? 

1875,  Nov.  1 1.  686  and  688  not  seen,  although  not  particularly  looked  for. 

1875,  Nov.  ii.  675  not  seen. 

1875,  Nov.  24.  647  >  (575)  >  (671)  or  (676)  >  589  >  (567). 

1876,  Jan.  3.  647  ;>  (575)  >  (671)  >  622  qr  625,  676?  647!     See  observations. 
1876,  Jaii.  4.  (671)  or  (676)  >  575  almost=:67i  >  622?  [or  625]  >  589  >  567. 
1876,  Jan.  4.  612  =  618  =  (676),  581  and  R.  56  both  seen;  also  636,  not  (654). 
1876,  Jan.  4.  709  rather  fainter  than  usual.     LASSELL'S  b  not  seen. 

1876,  Mar.  6.  (575)  >  589  >  (567),  622.     See  observations. 
1876,  April  i.   709  is  a  little  harder  to  see  than  the  5th  star  of  the  trapezium. 
[Query,  6th  star  ?  it  is  probably  the  6th  star.] 

1876,  April  T.  709  =  671  or  647  about.     It  requires  attention  to  see  it, 

1876,  Nov.  5.  675  visible.     (671)  >  676;  636  visible  but  not  (654). 

1876,  Dec.  5.  647X671),  (575). 

1876,  Dec.  5.  68 1  >  (676)  >  (651)  ;  not  much  difference  in  these. 

1876,  Dec.  5.  709  =  663  about. 

1876,  Dec.  31.  709  <657,  657  =  652  about. 

1876,  Dec.  31.  (671)  >  676,  (575)  =(589). 

1876,  Dec.  31.  647  and  671  >  (575)  and  (589)  although  647,  671,  and  (575)  are 
not  very  unequal.     686  seen  well. 

1877,  Jan.  2.  558  >  709  >  524,  (575)  =  671  =647  nearly. 
1877,  Jan.  2.  654  and  675  have  not  been  seen  since  Nov.  5. 
1877,  Jan.  10.  (575)  >  (641)  [just  seen]  >  (567)  [barely  visible]. 
1877,  Jan.  10.  567  =  i6th  mag.,  ARGELANDER. 

1877,  Jan.  24.  (642)  =  (654)  each  is  just  visible. 


l82  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

1877,  Jan.  24.   709  >  (641)  >  (676)  >  (567),  671  =622?  =  (575)  nearly. 
1877,  Jan.  24.  671  >  (589),  but  not  much.     (675)  not  visible;   581  >  573. 
1877,  Feb.  3.  (654)  and  (675)  just  barely  visible. 
1877,  Feb.  6.  651  very  faint.     675  seen. 
1877,  Dec.  7.  (602)  riot  seen,  589  >  567. 

1877,  Dec.  14.  (602)  not  seen. 

1878,  Jan.  3,  Jan.  5,  Jan.  6.  See  observations  with  aperture  of  3.50  inches. 
1878,  Jan.  5.  6.75  seen  following  frons  ;  L A SSELL'S  b  suspected;  6i2>6i8;  602 

seen  ? ;  654  not  seen  ;  567  —  16.3  mag. ;  686  and  688  not  visible.     It  not  seen. 

1878,  Jan.  16.    ioh;  654  seen!;  675  just  visible  following  the  frons ;  567  >  709; 
567  just  visible.     654  not  visible  at  nh. 

1878,  Jan.  24.   709  not  brighter  than  666.     771  >  676. 

1878,  Jan.  28.  8h  54m;  654  just  visible  — 618.     676  not  seen. 

1878,  Jan.  28.  Position  of  654,  ^a  =  +  8".4  (3),  4d  =  +  3i".7  (4) 

1878,  Jan.  28.   709.     See  observations. 

1878,  Feb.  4.  675  seen  about  on  edge  of.  frons  with  175.     LASSELL'S  b  seen! 

1878,  Feb.  4.  654  and  602  not  seen;  new  star  near  709?     a  not  well  seen. 

1878,  Mar.  9.  675,  b ;  a  seen,  Professor  HILL.     (See  observations.)     a  >  647. 

1878,  Dec.  6.  LASSELL'S  b  suspected  at  4b  25™  sid.  t. ;  not  visible  at  5h. 

1878,  Dec.  6.  654  and  675  not  to  be  seen. 

1879,  Jan.  10.  No  star  inside  the  trapezium  (HALL  and  HOLDEN)  ;  LASSELL'S  b 
seen  (HALL)  ;  also  a  star  in  _p  —  45°,  s  —  48"  from  708  (HALL). 

1879,  Mar.  i.  654  seen  (8h  40™) ;  b  seen;  h  visible;  651  visible. 

1879,  Dec.  26.  612  and  618  seen;  h  seen  like  a  nebulous  mass  and  not  like  a 
star ;  a  clear  dark  space  between  h  and  the  following  side  of  D. 

1880,  Jan.  3,  ioh;  675  seen  just  outside  of  pons  Scliroeteri ;  b  seen;   612  >  618  ; 
6i8>  567;  567  =  636  very  faint;  647  in  a  dark  space;  k  visible,  nebulous,  and  not 
stellar.     ioh  5™,  654  just  barely  visible. 

'  h  78  =  654. 

1874,  Jan.  1 6.  7h~7h  2om;  seen,  but  not  after  7b  30™. 
1874,  Jan.  17.  9h ;  not  seen;  (a)  seen  well;  636  seen  faint. 
1874,  Jan.  23.   ioh  ;  suspected,  but  too  cloudy  to  verify. 
1874,  Jan.  24.  9h  30™;  (a)  seen;  654  not  mentioned?! 
1874,' Jan.  25.  Not  seen;  (a)  seen. 
1874,  Feb.  5.  8h;  675  seen;  654  not  mentioned"?? 

1874,  Feb.  14.  Seen;  (602)  also  seen. 

1875,  Oct.  27.  I2h-i3h;  not  seen. 

1875,  Oct.  29.   I2b-i3h;   (654)  =  651  nearly. 

1875,  Nov.  5.   nb.5-i2h.5  ;  not  seen,  although  no  mention  is  made. 

1875,  Nov.  10.   i  ih.7~i3h.3  ;  not  seen  although  no  mention  is  made. 

1875,  Nov.  ii.  nh.7-i2h.5  ;  (654)  >  612  or  618,  (654) —622  >  621. 

1875,  Nov.  17.   I2h;  not  seen,  although  no  mention  is  made. 

1875,  Nov.  24.   ioh.2-nh.5  ;  not  seen,  although  no  mention  is  made. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  183 

1876,  Jan.  3.   i  ih.7  ;  not  seen,  although  no  mention  is  made. 

1876,  Jan.  4.   ioh.3  ;  not  seen;  612,  618,  and  636  seen. 

1876,  Jan.  10.   i  ih.5-i2h  ;  it  is  doubtful  whether  654  was  looked  for. 

1876,  Jan.  30.   ioh;  not  seen  although  no  mention  is  made. 

1876,  Feb.  30.  7h.5-8h.5  ;  it  is  doubtful  whether  654  was  looked  for. 

1876,  Mar.  6.  8h~9b;  not  seen,  although  no  mention  is  made. 

1876,  Mar.  14.  Not  seen,  although  no  mention  is  made. 

1876,  Mar.  22.  7h.7-8h.  It  is  doubtful  if  654  was  looked  for. 

1876,  Nov.  5.  654  not  visible,  although  no  mention  is  made. 

1876,  Dec.  5.  654  not  visible,  although  no  mention  is  made. 

1876,  Dec.  31.  654  not  visible,  although  no  mention  is  made. 

1877,  Jan-  2-  654  no*  visible,  although  no  mention  is  made. 
^1877,  Jan.  10.  654  not  visible,  although  no  mention  is  made. 

1877,  Jan.  24.  654  —  642. 

1877,  Feb.  3.  654  just  visible. 

1877,  Dec.  7-  654  not  visible,  although  no  mention  is  made. 

1877,  Dec.  14.  654  not  visible,  although  no  mention  is  made. 

1878,  Jan.  5.  654  not  visible. 

1878,  Jan.  1  6.  ioh;  654  seen?  not  seen  at  nh. 

1878,  Jan.  24.  654  not  visible,  although  no  mention  is  made. 

1878,  Jan.  28.  8h54m;  654-6i8.     Ja  +  8".4  (3)  Jd  =  +  31".;  (4) 

1878,  Feb.  4.  654  not  seen. 

1878,  Dec.  6.  654  not  seen. 

1879,  Jan.  10.  654  not  seen,  although  no  mention  is  made. 
1879,  Mar.  i.  8h  40™;  654  seen. 

1879,  Dec.  26.  8h  40m;  654  seen. 

1880,  Jan.  3.   ioh  5m;  654  just  barely  visible. 

With  regard  to  the  small  nebulous  mass  k  (at  first  supposed  to  be  a  star,  and  so 
marked  with  a  ?  on  my  observing  copy  of  the  Index-Chart)  I  have  to  say  that  I  am 
absolutely  certain  it  did  not  exist  in  its  present  form  from  1874,  January,  till  1878, 
January.  Since  this  time  it  has  been  constantly  seen,  and  is  growing  brighter.* 

Also,  it  may  be  recorded  here  that  the  nucleus  to  pons  Sckroeteri,  which  others 
have  called  stellar,  and  which  seemed  so  to  me  during  the  first  of  the  work,  is  not  so 
at  present  (1879-  1  880). 

COOPER'S  star  c',  near  516,  I  have  never  seen. 

LASSELL'S  double  star  6,  near  685,  certainly  exists.  It  is  very  faint.  have 
never  seen  his  star  g  (near  676),  and  I  doubt  its  existence.  These  stars  are  laid  down 
in  his  Plate  III  (and  p.  56)  of  Mem.  R.  A.  S.,  xxiii. 

The  others  I  identify  as  follows:  a  =  675;  k  =  671  and  676;  /  = 
1  =  612;  fc  =  6i8;  c  =  62i;  ^  =  625;  e  =  595  ;  /=  6o8- 

With  regard  to  675,  it  seems  to  me  that  this  extremely  faint  star  is  not  proved 


l&inch  CLAHK  telescope  of  the  Wa8hburn  Observa- 

tory,  although  I  have  repeatedly  looked  for  it  under  the  best  conditions. 


1 84 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


be  variable.  Atmospheric  changes  will  account  for  all  the  variations  I  have  observed. 
It  is  very  close  to  the  edge  of  the  regio  Huygheniana,  perhaps  even  within  it. 

709  is  certainly  variable  between  the  magnitudes  of  about  1 1.5  and  13.0  on  Gr.  P. 
BOND'S  scale. 

The  few  new  stars  I  have  myself  added  to  those  laid  down  by  BOND,  I  regard  as 
of  no  special  importance.  The  list  could  be  slightly  extended  if  it  were  desirable,  but 
not  more  than  five  or  six  stars  above  16.0,  ARGELANDER,  exist  in  the  limits  of  the 
Index-Chart  which  were  not  laid  down  by  BOND. 

The  minimum  'visibile  of  BOND'S  telescope  is  15.1  magnitude  (ARGELANDER'S  scale 
extended),  and  for  the  Washington  refractor  it  is  i6m.3. 

It  seems  to  me  that  the  fact  that  so  few  additional  stars  have  been  seen  is  of  great 
importance  in  throwing  light  on  the  question  of  the  depth  of  the  stellar  universe  in 
this  direction. 

It  will  be  interesting  to  know  if  other  large  telescopes  (as  Mr.  COMMON'S  new 
reflector)  can  detect  any  stars  fainter  than  675,  my  i,  2,  3  (near  663)  or  LASSELL'S  b. 
Lord  ROSSE  has,  I  believe,  but  one  star  (R.  56)  on  the  Index-Chart,  not  laid  down 
by  BOND. 

SYNOPSIS  OF  THE  MORE  IMPORTANT  MEASURES. 

From  the  preceding  synopsis  I  collect  the  most  important  measures  for  purposes 
of  comparison: 

Frons. 


Date, 

P 

No.  of  obs. 

Remarks. 

1875,  Oct.      29 

0 

48.3 

5 

Length  of  Frons  ;  1878,  January  16  ;  s  =  193".  5  (3). 

Nov.    ii 

52.1 

4 

r876,  Nov.     22 

49-3 

4 

. 

1878,  Jan.      16 

51.3 

4 

Adopted 

/=  50.3 

(17) 

' 

Occiput. 


Date. 

P  — 

No.  of  obs. 

Remarks. 

1875,  Oct.     29 

142.1 

5 

Perpendicular  distance  of  Occiput  from  01  =  105".  5  (3) 

Nov.    ii 

139-3 

4 

1877,  Jan.  5. 

1876,  Mar.      6 

142.7 

4 

(Nov.    22 
1877,5  T 
<Jan-       5 

[136.6] 
Ci35.5] 

(4)       ) 
(3)       5 

These  two  measures  cut  off  Y  altogether  and  give/)  = 
1  36°.  i  (7). 

1878,  Jan.      16 

140.1 

3 

Adopted 

p=  141.1 

(16) 

MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

y  (Sinus  Gentilii). 


1876,  Feb.  ?  North  end  of  Sinus,  J6-—  66.7  (i) 

1877,  Feb.  7.  JVortf*  end  of  Sinus,  46  =  —  66.7  (4) 

1878,  Mar.  5.  See  observations,  J6  =  —  58.3  (3) 
1878,  Mar.  5.  See  observations,  Jd  =  —  63.3  (3) 

Adojtted  J6  north  end  of  Sinus.  =  —  66.7  (5) 


185 


1876,  Feb.  ?  North  end  of  (5, 
1878,  Mar.  5.  AVM  end  of  6, 

Adopted, 


Dale.  p—  No. 


1875,  Oct.  2g  :     7.7  3 

1876,  Nov.  27     [5.1]  i 
1876,  Dec.  ig  [12.2 : :]  3 
1878,  Jan.  16      7.7  3 


Jd  —  —  102.6  (i) 
J6-—    99.2  (3) 

z/#  — —  100.9  (4) 
J  and  B. 


Adopted  \p--j.i  (6) 


Remarks. 


Angle  of  preceding  edges. 

Omitted  ;  the  first  depending  only  on  I  observation  ;  the 
second  is  doubtful.     See  observations. 


B. 


1877,  Dec.  12.  Tangent  to  the  north  side  of  the  curve  in  which   B  joins  A, 

+  3o".5  (4) 

W1  (center). 


Date. 


No. 


No. 


1875,  Nov. 

17 

-  37-0  : 

i 

1876,  Mar. 

14 

-  43.2 

i 

1877,  Nov. 

20 

1877,  Dec. 

7 

+  68.8 

1878,  Jan. 

23 

4-  67-7 

1878,  Mar. 

20 

-  47-4  :  : 

2      +  68.0 

1878,  Dec. 

6 

-  48.2* 

4 

Adopted      -  45-0  : 


(8) 


+  68.2 


'  edge  =  —  57  "-3  )  l» 

edge  =  —  29".!  )  (i) 


Wt.  =  2. 
Wt.  =  2. 

The  distance  from  the  center 
of  W  to  685  is  about  equal 
to  the  distance  635-669. 

*  Double  weight. 


The  distance  from  the  center  of  W1  to  684  is  about  equal  to  the  distance  635-669. 
From  the  shape  of  W,  the  Ja  of  the  cotter  is  necessarily  doubtful:   it  is  not  so 

with  the  JS. 

APP.  V 24 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

J. 

1878,  Mar.  20.  4a  of  the  point  D  of  LIAPONOFF=  preceding  point  of  J,  97". 3. 

A. 


Date. 


1876,  Feb.         ?  i 

1877,  Dec.        7  " 
Dec.      12 

Adopted 


A  J 


-  31- 


31.2 
31.6 


No. 


i 

2 

5 
(8) 


Remarks. 


Ad  south  end  of  A  near  622. 


I. 


1876,  Mar.      14     The  rounded  apex  (south  point)  is  bisected  in  A  a  =  —  2g."i. 

1878,  Jan.      23      s.  /.corner,  ad  =-  95".8  (i)j 

}  Adopted  Ad  =  —  96  .7  (2; 
1878.  Mar.        5      s.  /.  corner,  Ad  =  —  97".6  (i)> 

1878.  Jan.       28  corner  near  [A,],  Ad  =  — 77".4  (i)» 

C  Adopted  Ad  =  —  79  .3  (3) 
1878,  Mar.        5  corner  near  [Ai],  Ad  =  —  81   .2(2)) 

1878,  Dec.        6  corner  near  [Ai],  A«  =  —  62''. 8  (4) 


L  (north  end]. 


Date. 

Ad 

No. 

Remarks. 

1876,  Feb.         ? 
1877,  Dec.      12 

—  25.6 

-  25.4 

, 

4 

On  same  parallel  as  671. 
Near  parallel  of  617  and  622.     (See 

observations.) 

Adopted  Ad  = 

-  25.5 

(5) 

(See  observations.) 

E. 


Date. 

South  point. 

A<5 

No. 

North  point. 
A  '5 

No. 

Apex. 
•    Aa 

No. 

1875,  Nov.      10 

-   147-6 

3 

—   101.7 

5 

" 

Nov.      17 

.      . 

•     . 

0.0 

1876,  Feb.         ? 

—   144.0 

*j 

—    IO2.6 

.1 

. 

Mar.       14 

. 

5-3* 

i 

1877,  Feb.        6 

. 

. 

+  2",  +  3"  (est.) 

. 

Dec.         7 

-   143-5 

4 

. 

. 

Dec.        7 

-   I3i-4t 

I 

. 

Dec.       \2\ 

-   155-  6§ 

2 

.     . 

-    . 

1878,  Jan.        28 

-   147-2 

3 

.     . 

. 

1879,  Jan.       20 

-    96.7 

2 

•      • 

Adopted  Ad  = 

-   144.6 

(16) 

—  100.3 

(8) 

From  +  o"  to  +i"  (est.) 

*  Poor  observations. 

fThis  is  the  extreme  point  towards  the  north  which  could  be  taken  as  the  vertex. 

S  This  is  the  extreme  limit  of  E  towards  the  south.     The  mean  of  these  is  about  the  true  Ad. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Dark  channel  between  I  and  (X  and  G). 
1876,  Mar.  6.  ^  =  25°. 6  (3) 

Dark  channel  between  I  and  E. 


187 


Date. 


1876,  Dec.        5  42.6 

Dec.      31          [50±] 

1878,  Jan.        16  46.0 


Adopted  /=44.o 


No. 


4 

est. 
i 

(5) 


Remarks. 


Uncertain. 


Dark  channel  between  E  and  F 


Date. 


No. 


Remarks. 


i875,Nov.      24       /  =  I39.6  2  It  passes  through  the  star  580^  1875,  Nov.  24. 

1878,  Jan.       16    .  ^  =  142.1  3 

Adopted      /  =  140.9  (5)- 

Dark  channel  between  F  and  (G  and  H). 

Its  prolongation  is  in  the  line  685-708,  from  many  measures. 

V. 

Extreme  north  point  in  same  J&  as  612;  i.  e.,  Jd  —  -\-  24". 6;   J6  south  edge  of  n. 
•e  of  M,  JS—  —  22". 7  (3) ;  good. 

F. 


Date. 

Brightest  part.                        North  point. 

South  point.            Middle  point. 

A« 

No.            A<5 

No.         A'5 

No. 

A1*           No.           A(»            No. 

—   122.3           i               ... 

.      .            .          +  28.8           i 

1875,  Nov.^io 
1876,  Feb.         ? 
Mar.      14 

•       • 

—    IOI.O 

. 

3             •    • 

• 

1877,  Feb.         7 
Dec.       12 

—    102.1 

4 
-  94-9 

2 

•   i 

-    II7-3*            2 

1878,  Jan.       28 
Feb.      20' 

+  26.8          2               -    102.9 

4 

—    120.6               I                    . 

Mar.        5 
1879,  Jan.       20 

Adopted 

+  26.8 

—     IOI.2 
—    102.8 

2                   .     . 

4 
(I?)        -   94-9 

. 

[  —  113.5]!         .                ... 

-     120.  I             (4)            +    28.8            (I) 

(2)              -    102.0 

(2) 

*  In  Frons.         fThis  is  the  s.p,  corner  ;  see  observations. 


I  88 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


1876,  Mar.  14.  Tangent  to  preceding  sides  of  F  and  G,  z/#  —  -f  i"-2. 

1877,  Feb.  6.  Point  where  s.  p.  edge  intersects  frons,  J6  =  —  24".  6  (3) 

1878,  Feb.  20.  The  bright  part  of  F  first  begins  to  be  fainter  at  Jd  —  —  1  14."  9. 

1879,  Jan.  20.  The  north  point  of  X  (F)  is  further  north  than  JS  —  —  97". 

G. 


Date. 

Brightest  part. 

Middle  point. 

Remarks. 

A  a        No.         Ad 

No. 

2 

Aa 

No.        Ad       No. 

1875,  Nov.      10 

. 
.      .           .       -64.5 

" 

" 

Tangent  to  following  side  of  G,  1876, 
March  14;   A"  =  +  28".8. 

1876,  Feb.         ? 

.      .           .       -69.6 

I 

• 

—  69.6        I      Tangent  to  preceding  side  of  G,  1876, 
Mar..  14;   Ao=-t-i".2;  1877,  Feb. 
6,  about  o"  ±  . 

Mar.      14 

. 

+  14-2 

Dec.        5 

.       -65.5 

5 

1877,  Feb.        3 

. 

-M3-9:       3 

Feb.        7 



• 

—66.2       4      South  point;   A<J;=  —  75".  7  (4)  ;  1878, 
Jan.  28. 

Nov.     20 

.      .           .       —60.9 

3 

.      .'         .        .      .          .      1878,  March  5.     All  of  G  is  north  of 
Ad  =-77". 

1878.  Jan.       28 

+  15.9       3      -66.5 

3 

1879,  Jan.       20 
Adopted 

.      .          .      —64.9 

(4) 

+  15.9      (3)    -65.3 

(18) 

-t-14-r 

(3)      -67.9     (5) 

H. 

1877,  Dec.  12.  JS  of  south  end  of  H  [in  frons]  — —  94". 2  :  (i)  )  these  not  com- 

1878,  Mar.  5.  Jd  of  south  edge  of  H  along  channel  —  —  77.5  (3)  )       parable. 

D. 

n.f.  point.       Following, 


Date. 

LUgC. 

Ad 

No. 

Aa 

No. 

1875, 

Nov. 

10 

+  77-7 

3 

" 

1876, 

Mar. 

M 

. 

+28.8 

I 

Dec. 

19 

.       .             .        +28.8 

1877, 

Feb. 

3 

. 

+30.5 

3 

Nov. 

20 

Si  :: 

. 

. 

1878, 

Jan. 

28 

.      .          .      +29.6       3 

Dec. 

6 

.      . 

+29.3 

4 

Adopted  . 

+  78          (3) 

+29.4  (II) 

MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Pons  Schroeteri  and  g0. 


Date. 


1875,  Nov.  10     .        . 
Nov.  ii  . 
Nov.  17      +     79.3 

1876,  Feb.  ?  .      . 
Mar.  14      +     76.8      (i) 
Dec.  19 

1877,  Jan.  5 
Jan.  10 

Feb.  3      +     77.4 

Nov.  20 

Dec.  12  .   i 

1878,  Jan,  16  .      .        . 
Jan.  28      +     76.6  I     3 
Dae.  6      +     76.7 


go 
No. 


Adopted       .      +     77.4 


(15) 


A(5        No. 


No. 


170.7  3 

175  ±  est.      . 


+     39-5        5 


+     41-9 


175-8 

+       41.6        (2) 
+       40.3        (4) 

.      .        .      167.5:  3 


+      40.8     (I2)|    170.2  (14) 


Remarks. 


See  observations  1876,  Feb.  2. 


(Uncertain.) 


M. 

1878,  Mar.  5.  North  edge,  Jd—^—221'^  (3) 

6  (Spitee). 


Date. 


1875,  Nov.      10 


Spitze. 
Aa          No.          A(5          No. 


Remarks. 


_l_     78.4     .4      1875,  Nov.  ii,  south  shore  a  ;  ^  =  89°.  7  (4) 


Nov.  17   +169.9*  3 

,  1877,  Feb.  3  j  +  165.1   3 

Feb.  7 

Dec.  12 

1878,  Jan.  23 


.      .        .      1877,  Jan.  30.  south  shores;  through  tour- 
malines to  compare  with  LE  GENTIL  /  = 
44*  (i) 
.     .        .      1876,  Feb.?  south  shore  <r  near  pons  S,  Ad 

=  +  83".?. 

+     75.7        4      South  edge  of  point. 
-I-     79.6       4     See  observations. 
.    [+     68::        i]    South  edge. 


Jan.       28 
Dec.        6 


+   162.8       3      +     80.7       4 
+   161.3       4 


Adopted       .   |  +   164.8    (13)     +     78.6    (16)      *Extreme   point. 


189 


Q,  P,  R- 

1875,  Nov.  n.  The  line  of  their  n.  f.  sides  passes  through  654  and  624  nearly 
Same  date  p  —  105°. 8  (4)  ;  this  cuts  off  some  of  R. 


878,  Jan.  28.  South  point  in  frons, 
878,  Mar.  5.  SoM  point  in  frons, 


-  74".  2  (2) 

-  75".o  (i) 


190 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OE  THE  NEBULA  OF  ORION. 


Q. 


Date. 

Following  point. 

Remarks. 

Aa 

No. 

Ad 

No. 

1875,  Nov.  .   17 

4-   150.6 

i 

" 

Masses  inside  of  Q  are  called  n  and  q. 

1876,  Feb.         ?i           .      . 

—     25.6 

i 

North  end  of  n  ;  A<5=  —  35".  5  (i) 

1877,  Feb.    "    3      4-   147.3 

3 

. 

Middle  of  q;   A  <5=  —  23".4  (i) 

Feb.        7 

. 

—     31.6 

4 

Nov.      20 

. 

-     25.7 

3 

Dec.      12 

1878,  Jan.       28 

4-   152.6 

4 

-      29-9 

3 

Bisecting   the  following  pointAJ=  — 
29"-9  (3) 

Adopted 

+   150.2 

(8) 

—      28.2 

(») 

on. 


1876,  Feb.  1  The  parallel  ^tfrz-f- 60"  passes  through  the  center  line  of  the  west 
half  of  on. 

Sinus  magnus. 

1876,  Nov.  1 1.  Best  tangent  to  south  shore,  p  —  105°. 6  (4) 


Although  a  comparison  between  the  measures  of  LIAPONOFF  and  myself  is  no, 
very  valuable  on  account  of  the  different  brightness  of  the  nebula  to  the  two  telescopes, 
I  have  added  a  short  catalogue  of  our  measures  on  the  same  points.  Other  comparisons 
have  been  given  in  the  preceding  sections. 

Comparison  of  positions  of  nebulous  masses  determined  at  Kasan  and  Washington. 


n.  f.  pt.  J  = 


Kasan. 

Washington. 

A        A       n        n 

lass. 

A. 

D. 

Aw. 

Dw. 

w  —  A.        L»w  —  U. 

Remarks* 

„ 

„ 

a 

„ 

U                                           II 

[D]      -     94-3      - 

39-9      -     97-3 

-          3-0                    .        . 

[Ai] 

-     61.5      - 

76.4             62.8 

-     79-3 

4-         1.3       -         2.9 

[T] 

-      35-2       4- 

22.  0 

4-     24.0 

4-         2.0 

[A] 

4.4       — 

138.0      4-o"  +i" 

-   144.6 

4-       3.9      —       6.6 

[*>] 

4-      18.5      - 

63.3      4-     15-9 

-     65.3 

—          2.6        —          2.O 

Brightest  point. 

[F] 

4-26.5       4- 

69.6 

4-     29.4 

4-        2.9                .       . 

[«o] 

4-      28.6       - 

97-4 

4-     26.8 

—    IO2.O 

1.8               4.6 

Brightest  part. 

l>o] 

+      71-9       + 

42.3 

+     77-4 

4-      40.8 

4-       5-5                1-5 

[K] 

-t-    143.6       4- 

71.5  :  4-  164.8 

4-      78.6 

4-      21.2        4-         7-1 

[B] 

4-    146.5 

22.  O       4-    150.2 

—      28.2 

4-         3.7                    6.2 

NOTE. — The  letters  in  [  ]  give  LIAPONOFF'S  nomenclature. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  191 

REDUCTION  OF  PHOTOMETRIC  OBSERVATIONS  MADE  AT  WASHINGTON. 
§  i.  Results  from  eye  observations  of  order  of  brightness. 

In  making  the  eye  estimates  of  the  order  of  brightness  of  the  various  bright 
masses  of  the  Huyyhenian  region,  it  was  the  object  simply  to  give  the  order,  without 
attempting  to  assign  how  much  brighter  one  was  than  another.  Again,  as  these  masses 
have  a  sensible  magnitude,  it  was  impossible  to  estimate  the  effect  of  the  mass  as  a 
whole,  and  to  compare  it  with  the  total  effect  of  another  mass,  perhaps  three  minutes 
of  arc  distant.  Hence,  these  estimates  must  be  understood  to  refer  to  the  brightest 
parts  only  of  each  mass,  and  they  give  the  facts  as  faithfully  as  was  possible.  "F  is 
brighter  than  H,"  for  example,  means  that  after  carefully  examining  these  two  masses 
I  believed  that  there  was  a  considerable  portion  of  the  surface  of  F  which  was  brighter 
than  any  considerable  portion  of  the  surface  of  H.  Again,  1876,  January  4,  I  esti- 
mated the  order  of  brightness  as  follows : 

.      i.  A.     2.  D.     3.  G,  H,  N.     4.  F.     5.  I,  Q.     6.  E.     The  numbers  i 6 

simply  indicate  the  order,  and  it  must  not  be  supposed  that  the  ratio  of  A  to  D  is  the 
same  as  that  of  D  to  G,  or  that  the  steps  are  equal. 

The  difficulty  in  obtaining  the  relative  brightness  of  the  various  masses  from  the 
observed  sequences  follows  chiefly  from  the  want  of  a  numerical  ratio  between  the 
different  steps,  and  also  from  the  fact  that  the  possibility  remains  that  different  portions 
of  a  certain  mass  are  taken  at  different  times,  as  representing  the  mass.  These  two 
objections  are  inseparable  from  the  method,  and  perfectly  definite  results  can  only  be 
obtained  from  some  form  of  photometer.  If  a  drawing  could  be  made  each  night 
which  would  represent  the  various  portions  of  the  nebula  as  they  appeared  on  that 
night,  and  if  such  drawings  could  be  continued  without  bias  or  prejudice  from  night 
to  night,  then  it  is  possible  that  increased  accuracy  might  result,  as  something  like  a 
numerical  ratio  of  the  brilliancy  of  the  various  masses  might  be  obtained.  But  this 
process  is  impracticable,  first,  because  the  nebula  is  too  large  to  sketch  carefully  in 
one  night;  and  secondly,  because  after  becoming  familiar  with  its  parts  it  is  extremely 
difficult  to  avoid  (in  drawings)  inserting  the  results  of  past  observations  instead  of 
present  appearances. 

The  published  drawings  of  various  astronomers,  in  so  far  as  they  relate  to  the 
order  of  brightness  of  the  various  masses,  are,  in  general,  but  imperfect  evidence. 
The  final  drawing,  which  has  probably  required  weeks  to  make,  may  be  said  in  general 
to  give  the  average  forms  of  masses  and  parts  which  are  well  seen,  with  accuracy,  and 
the  forms  of  the  fainter  masses  will  also  approach  to  correctness.  With  the  bright 
of  these  masses  it  is  different;  the  completed  drawing  gives  the  average  Appearance 
to  the  observer  so  far  as  his  artistic  skill  enables  him  to  render  it,  together  with  a 
number  of  imperfect  or  even  false  representations  due  to  the  imperfections  ,,f  the 
telescope,  etc.,  the  state  of  the  atmosphere,  the  bias  or  prejudice  of  the  obsen -.-,• 
(which  mostly  is  due  to  his  good  remembrance  of  previous  views  of  the  object  in 
question),  and  above  all  to  the  shortcomings  of  the  engraver. 

In  Part  III,  I  have  more  particularly  considered  the  value  of  the  evidence  which 
may  be  had  from  drawings,  and  I  have  suggested  the  use  of  a  terrestrial  eye-piece  M 
a  check  upon  this  kind  of  work. 


192 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


In  the  sequences  of  brightness  here  given,  I  endeavored  to  avoid,  as  far  as  pos- 
sible, all  bias  or  prejudice  by  not  collecting  and  examining  the  results  of  observations 
until  some  two  years  had  elapsed  since  beginning  the  work. 

The  examination  of  the  sequences  that  follow  raised  a  strong  suspicion  of  varia- 
bility among  some  of  the  bright  masses  of  the  Huyghenian  region,  and  to  get  further 
light  upon  this  the  photometric  observations  of  §2  were  made.  The  eye  observations, 
however,  are  independent  and  may  be  considered  by  themselves. 

Table  shouting  order  of  brightness  of  the  various  masses  (by  eye  estimates). 


"o 

Date. 

Decimal 
a  year. 

I                      2 

3 

4                  5                678 

' 

9 

Remarks. 

1875. 
Oct.     27 

.82 

A              D 

F  =  G 

I            N=Q      H=E 

See  observations. 

29 

•  S3 

A              D 

Fand  G 

I           N  and  O        E 

29 

•83 

A         F,  G,  D 

I 

H                N          Q  =  E 

Perhaps  better  than  last 
preceding. 

Nov.      5 

.85 

A              D 

FandG 

H  and  I        N,  Q            E 

• 

Only  brightest  parts  of  N 
and  O  included. 

10 
10 

.86 
.86 

A              D 
A              D 

F,G 
G,  F 

I,  H            N,Q           E 
I,  H            N,  Q            E 

• 

t  .  •  .  G  =  F. 

17 

.88 

A              D» 

G 

F             I  =  H          N           Q        E 

24 

.90 

A              D 

G=H 

F                N              0           I      E&J 

1876. 
Jan.       3 

.01 

A              D 

G,H 

N,O,  F,  I         E 

4 

.01 

A              D 

G,  H.N 

F              I,Q            E            . 

I  =  Q  nearly. 

10 

•03 

A              D 

G,  H,N,Q 

F               I,  E           .... 

Poor  obs'n  ;  moonlight. 

30 

.08 

. 

.             .... 

E  is  the  faintest  mass  of  the 

principal  ones. 

Feb.       ? 

.09 

A              D 

G,  H.  F 

N              1,0             E 

.. 

5-     I.  Q  or  Q,  I. 

Mar.      6 

.18 

[A              D 

F,  G,  H 

N                 I           O,  L] 

"  Not  verv  satisfactory"  ; 
rejected. 

6 

.18 

A              D 

F 

G,I           H,  E         N,Q 

This  is  better. 

22 

.22 

A              D 

F 

G,  I,  H       N.Q.E       .      . 

Approximately. 

Nov.    ii 

.86 

. 

H.G.F* 

I                E 

*About  equal. 

Dec.    13 

•95 

A              D 

G.  H,  F 

N,  I.  E            .                .             .           . 

"  Mr.  H.  S.  Pritchstt  thinks 
D  >  A  a  little." 

19 

•97 

D  =  R          A 

I 

H,F              B 

. 

Mr.  Pritchett. 

19 

•97 

A              D 

F 

Q                N               G          H          I 

E 

B=F=W;  y  =  o;  y  =  a 
near  K. 

1877- 
Jan.        2 

.00 

A              D 

G,  F 

H,I,N            E           [M            a         //] 

. 

.  J  <  E,  J  XT 

1878. 
Ian.     28 

.08 

• 

•      • 

G  >  F. 

For  observations  of  relative  brightness  by  means  of  tourmaline  plates  see  obser- 
vations of  1877,  Jan-  24»  Jan-  3°»  and  Feb.  3. 

1878,  Jan.  23.  F,  G,  H,  and  two  surfaces  in  P  ?  and  Q  are  brighter  than  I  or  K. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  193 

1878,  Dec.  ii.  F  —  G  -  H ;  perhaps  F  is  a  little  brighter  than  G  or  H,  but  this 
is  doubtful. 

1879,  Dec.  26.     i.  A,  I).     2.  G,  H.  F,  I,  Q,  E,  G,  and  H  notably  brighter  than  F. 

Discussion  of  the  Results  from  the  preceding  Table. 

It  will  be  convenient  to  consider  the  masses  in  pairs,  and  the  following  nomen- 
clature is  used.  G  =:  F  means  that  these  masses  were  considered  equal.  G  >  F  that 
one  was  decidedly  brighter  than  the  other,  and  that  they  are  in  different  compart- 
ments of  the  table.  G1  =  F  means  that  they  are  in  the  same  compartment  of  the  table, 
but  that  G  is  written  first  and  was  brighter,  though  not  greatly  so.  This  nomencla- 
ture was  used  during  the  observations  themselves. 

• 

A  and  D. 

From  my  own  observations  from  1875  to  1877,  January,  the  result  is  that  A  >  D. 
By  this  is  meant  that  the  bright  strip  on  the  following  edge  of  A  is  brighter  than  any 
considerable  part  of  D.  It  will  be  noticed  that  Mr.  PRITCHETT  makes  D  >  A.  There 
is  little  doubt  but  that  this  is  erroneous. 

F  and  G. 
\ 

The  following  are  the  separate  inequalities,  the  last  line  showing  the  means: 


Date  :    Ratio. 

Date  :    Ratio. 

Date  :     Ratio. 

Date  :     Ratio. 

Date  :    Ratio. 

Date  :     Ratio. 

Date  :     Ratio. 

i875.82,F=G 
75.83)F=G 
75.85,F=G 
1875.86,  F=G 

1875.88,  G>F 
75.90,  G>F 
76.01,  G>F 
76.01,  G>F 
i876.03,G>F 

1876.09,  G!  =  F 
*i876.i8,  F'=G 
*Unsatisfactory. 

1876.18,  F>G 
1876.22,  F>G 

1876.86,  G'  =  F 
1876.95.  G'  =  F 

1876.97,  F  >  G 

1877.00,0'  =  F 
1878.08,6  >  F 

1875.84,  F=G 

i875.g6,G>F 

1876.09.  G'  =  F 

i876.20,F>G 

i876.9i.G'  =  F 

1876.97,  F>  G 

1877.00,  Gl  =  F 
1878.08,6  >  F 

Following  the  last  line  of  this  table,  it  appears  that  1875.84  F  and  G  were  equal; 
G  was  brighter  than  F  a  little  later,  then  passed  through  equality  with  F  to  G  <  F. 
A  similar  consistent  fluctuation  is  shown  from  1876.91  to  1878.08. 

It  should  be  noted  that  in  the  above  table  there  are  no  cases  like  this :  F  >  G, 
F  =  G,  and  then  F  >  G ;  and,  again,  there  are  no  cases  like  F  >  G,  and  then  G  >  F 
without  passing  through  F  —  G.  These  results,  taken  alone,  appear  to  indicate  fluc- 
tuations in  the  relative  brilliancy  of  F  and  G.  Whether  these  are  irregular  or  periodic 
is  not  to  be  decided  by  observations  of  this  nature,  but  it  will  require  a  careful  and 
long-continued  series  of  photometric  experiments  conducted  with  every  possible  pre- 
caution against  systematic  errors  and  against  preconceived  opinions  and  bias. 
25 


I94 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

I  and  H. 


1875.82,!  >  H 
1875.83,!  >  H 


1875.83,!  >  H 


1 875. 85,  H  =1 
1875.86.11   =H 
1875.88,!    =  H 


1875.86,  H  = 


1875.90,  H  >  I 
1876.01,  H  >  I 
1876.01,  H  >  I 
1876.03,  H  >  I 
1876.09,  H  >  I 


1876.18,    I  >  H 


1876.01,  H  >  I         1876.18,    I  >  H 


1876.22,  I1  =  H        1876.86,  H  >  I    I     1876.97,    I>H       1877.00,  H'=  I 


1876.95,  H  >  I 


1876.22,  H  =  H        1876.91,  H  >  I 


1876.97,  H  >  I 


18.6.97,  H  =  I?       1877.00,  H'=  I 


This  table  is  not  so  clear  as  the  preceding  one.     The  comparison  was  more  diffi- 
cult.    It  seems  to  me  to  indicate  changes,  however. 

N  and  Q. 


i875.82,N  =  Q 

1875.83.  N  —  Q 

1875.83,  N  >  Q 

1875.85,  N'  =  Q 
1875.86,  N1  —  O 

1875.88,  N  >  Q 
1875  90  N  >  Q 

1876.01,  N'  =  Q 

1875.86,  N'  =  Q 

i875.83>N  =  Q 

1875.83,  N  >  Q 

1875.86,  N!  =  Q 

1875.89,  N>  Q 

1876.01,  N!  =  Q 

1876.01,  N  >  Q 

1876.03,  N1  =  Q 

1876.09,  N  >  Q 

i876.i8.N»  =  Q 
1876.22,  N'  =  Q 

1876.97,  Q  >  N 

1876.  01,  N>  Q 

1876.03,  N'  =  Q 

1876.09,  N  >  Q 

1876.20,  N1  =  Q 

1876.97,  Q  >  N 

As  the  difference  between  Q  and  N  in  any  event  is  but  small,  the  above  com- 
parisons, taken  alone,  indicate  that  N  —  Q,  and  that  the  variations  of  the  above  table 
are  accidental. 

I  and  Q. 


Date:       Ratio. 

Date  :       Ratio, 

Date:       Ratio. 

Date  :       Ratio. 

1875.82,  I  >  Q 

1875.90,  I  <  Q          1876.18,  I  >  Q 

1876.97,  I  <  Q 

1875.83,  I  >  Q 

1876.01,  I  =Q 

1876.22,  1  >  Q 

1875.83,  I  >  Q 

1876.03,  I  <Q 

1875.85,  I  >  O 

1876.09,  I  =  Q 

1875.86,  I  >  Q 

1875.86,  I  >  Q 

1875.88,  I  >  Q 

1875.85,!  >  Q 

1876.0  I?=Q? 

1876.20,  I  >  Q 

1876.97,  I  <  Q 

It  is  possible,  almost  probable,  that  variations  in  brightness  are  here  indicated, 
taking  these  observations  alone. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

I  and  N. 


195 


Date:       Ratio.         Date:       Ratio.         Date:       Ratio.         Date:       Ratio.         Date:       Ratio. 


1875.82,  I  >  N 
1875.83,  I  >  N 
1875.83,  I  >  N 

1874.90,  I  <  N 

1876.01,  I  =  N 
1876.01,  I  <  N 

1876.18,  I  >  N          1876.95,  I  <  N 
1876.22,  I  >  N          1876.97,  I  <  N 
1877  oo    I  —  N 

1875.85,  I  >  N 

1875.86,  I  >  N 
1875.86,  I  >  N 
1875.88,  I  >  N 

1876.03,  I  <  N 
1876.09,  I  <  N 

1875.85,  I  >  N 

1876.01,  I  <  N 

1876.20,  I  >  N          1876.96,  I  <  N 

1877.00.  I  =  N? 

Taking  the  tables  N  and  Q,  I  and  Q,  and  I  and  N  together,  it  would  seem 
pretty  certainly  that  Q  —  N,  and  that  variations  in  I  place  it  sometimes  brighter  and 
sometimes  fainter  than  Q  and  N.  These  may  vary  also,  but  if  they  do  they  vary 

together. 

E. 

E  is  pretty  uniformly  the  faintest  of  the  principal  masses  (A,  D,  E,  F,  Gr,  H,  I, 
N,  and  Q).  It  is  to  be  noted  that  the  position  thus  assigned  to  E  in  order  of  brightness 
is  different  from  that  of  the  older  drawings,  1837-67.  My  observations  since  1878.0 
indicate  that  E  was  brighter  than  before  this  date. 


E  and  J. 


1875.90.  W—  J  -,   1877.00,  J  <  E. 


1876.97-  Y  —  ff,  and  Y  —  o  ,  •  .  Y  zz  0  —  a. 
1877.00.  E  >  J  >  ff,  and  M1  zz  ff. 

1877.00.  ff1  zz  y". 

Probably  the  foregoing  observations  are  not  in  themselves  sufficiently  exact  to 
merit  any  better  reduction.  Without  giving  them  any  great  weight  they  still  seem  to 
show  that  several  of  the  masses  of  the  central  region  are  subject  to  variations  of  bright- 
ness. The  periods  of  these  and  the  amount  of  change  cannot  be  deduced, 
add  to  the  above  recorded  observations  my  strong  impression  that  in  the  years 
1878-1879  the  mass  D  was  fainter  relatively  than  formerly,  and  the  mass 

§  2.  OBSERVATIONS  WITH  HASTINGS'S  PHOTOMETER. 

In  view  of  the  foregoing  results,  I  requested  Dr.  CHARLES  S.  HASTINGS,  of  tin- 
Johns  Hopkins  University,  to  devise  for  me  some  form  of  photometer  for  mea 
the  suspected  changes,  which  he  kindly  did.     The  conditions  were  somewhat  roi 
as  the  filar  micrometer  could  not  be  removed  to  insert  a  photometer  without      angr 
the  zero  of  position,  which  was  undesirable,  and  therefore  some  form  had 
which  could  be  inserted  beyond  the  micrometer  box. 


196  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

The  form  finally  chosen  was  as  follows  (see  Fig.  39,  which  is  drawn  to  halt  the 
natural  size): 


FIG.  39.     Dr.  C.  H.  HASTINGS'S  nebula  photometer. 

A  terrestrial  eye-piece  I,  II,  III,  IV  was  screwed  in  the  micrometer  box  in  the 
same  thread  as  the  ordinary  micrometer  eye -piece,  which  was  removed.  The  principal 
focus  of  the  objective  is  at  the  4.  in  the  figure.  A  and  B  are  diaphragms.  C  D  is  the 
side  of  the  eye-piece  tube.  In  the  focus  of  IV  was  placed  a  silver  mirror,  m,  which 
was  illuminated  by  a  movable  lamp,  m  was  semi-circular  in  shape,  and  about  15"  on 
the  straight  edge  (diameter).  A  screen  was  fixed  in  position  outside  the  tube  C  D. 
This  screen  was  covered  with  a  bit  of  paper  cut  from  BOND'S  engraving  of  the  nebula 
(see  frontispiece).  It  was  illuminated  by  the. (movable)  lamp.  In  looking  into  the 
photometer  a  semi-circle  (m)  of  precisely  the  same  color  as  the  nebula,  was  seen  illumi- 
nated by  the  lamp.  This  semi-circle  was  projected  upon  the  nebula,  and  to  measure 
the  brightness  of  any  part,  as  E,  for  example,  the  m  was  placed  next  to  the  brightest 
part  of  .E,  and  the  brightness  of  m  changed  by  moving  the  lamp  until  it  matched  E 
or  until  the  line  of  separation  vanished. 

This  was  done  sometimes  by  increasing,  sometimes  by  diminishing  the  brightness 
of  m.  The  position  of  the  lamp  relative  to  the  screen  was  read  off  on  the  graduated 
rod  that  bore  it.  These  are  the  essential  features  of  the  apparatus.  The  observations 
were  never  easy.  The  chief  difficulties  with  this  apparatus  were  first,  that  the  illumi- 
nation from  the  small  lamp  was  never  constant,  and  I  had  continually  to  return  to  the 
mass  D,  which  I  used  as  a  zero  mass;  and  second,  there  was  a  difficulty  in  being  sure 
of  the  exact -portion  of  the  surface  of  D  used  as  a  zero.  That  is,  there  was  a  possibility 
of  using  different  parts  of  D  for  zeros  at  different  hour-angles.  Both  these  difficulties 
were  purely  mechanical  and  could  easily  be  overcome,  but  it  was  impracticable  to  do 
this  under  the  actual  circumstances.  The  results  are  somewhat  less  consistent  on  these 
accounts,  but  after  all,  systematic  errors  are  more  to  be  feared  here  than  comparatively 
small  accidental  ones. 

If  d  is  the  reading  of  the  graduated  rod  for  the  zero  mass  D,  and  if  e  is  the  read- 
ing for  any  other  mass,  as  E,  then 

i     i  d2 

brightness  of  E;  brightness  of  D  zr  -* -:  ,9  or  E  =:  D. 

e2   d2  e2' 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


'97 


Following  I  give  the  reducti6ns  of  all  the  photometer  measures  made. 
The  reductions  previously  given  in  the  observations  of  each  night  were  prelimi- 
nary, though  only  slightly  different  from  the  ones  here  set  down. 

Collection  of  single  results  of  photometer  observations. 


Mass. 

Date. 

A. 

F. 

G. 

[G]. 

H. 

I. 

E. 

Q. 

f- 

1878. 
Jan,       7] 

I.42*/ 

T      1C*f 

\ 

12 
26 
26 

Feb.      4 

1  O5    > 

I.  12 
1.03 

i.  08 

1.  00 

1.09 

•     •- 

•     • 

•    • 

o.gof 
0.84 
0.77 

[0.55] 
see  obs. 

•    • 

0-43 
0.39 

5 

1.71 

1.05 

1.  14 

•     . 

•     . 

0.75 

0.83 

0.83 

. 

5 

•   • 

1.  12 

I.  21 

. 

0.79 

0.88 

0.88 

. 

5 

•   • 

•       • 

0.94 

•     • 

•     • 

•     • 

0.74 

•     • 

•     • 

[D]f 

26 

.   . 

O.gi 

1.26 

i.  02 

I.I? 

0.99 

0.92 

1.30 

. 

26 

.   . 

0.82 

i.  A 

0.92 

i.  06 

0.89 

0.83 

I.I? 

. 

26 

. 

. 

.     4 

. 

.     . 

. 

26 

(By  eye) 

F>G 

G>H 

. 

I  >  E 

. 

. 

. 

28 

(By  eye) 

A>  D 

F  >  G 

G  =  H;  I>E 

. 

.     • 

. 

.      . 

. 

28 

. 

1.23 

. 

I.OI 

0.91 

0.83 

i.iS 

. 

Mar.      9 

[I.I5] 

1.04 

[o.82]§ 

. 

1.05 

0.78 

.     . 

9 

•      • 

1.90 

0.90 

[0.71] 

.     . 

0.91 

0.67 

.     . 

9 

.      . 

[0.84]  part. 

.     . 

.     . 

.     . 

0.99 

0.91 

.     . 

9 

•      • 

[0.73]  part. 

•      • 

•     • 

0.86 

0.79 

•     • 

•      • 

1879. 
Feb.     23 

> 

0.98 

Mar.      7 

1880. 
Jan.       3 

2.25  :: 
see  obs. 

1.27 

Vj 

[I  =  o.  91  E] 
[I  =  o.82  E] 

0.81 

I    •  • 

3 

. 

. 

1.18 

.     . 

1.07 

0.88 

1.07 

•      • 

•     • 

10 

. 

I.  10 

1.23 

. 

. 

.     . 

o.73 

•      • 

•     • 

10 

. 

i.  or 

1.07 

•     • 

•    . 

0.62 

•      • 

•      • 

10 

. 

0.97 

. 

.      . 

.     . 

•     • 

13 
14 
14 

•     • 

1.52 
1.23 

1.42 

•     • 

0.90 

0.93 
i.ig 

i.  06 
0.87 
i.  08 

•      • 

•      • 

16 

. 

1.28 

I-3I 

1.04 

1.  12 

•     • 

16 

•     • 

•     • 

•    '  • 

•      • 

1.14 

1.04 

• 

• 

*  Preliminary  observations. 

fThe  brightest  part  of  D  compared  with  the  standard  part  of  D. 

\  This  is  not  the  same  part  of  A  as  Jan.  7.     [G]  is  a  part  of  G  defined  in  obs.  Feb.  26.  q.  v.;     D]  is  the  brightest  part 
of  D,  not  the  part  used  as  the  standard  of  comparison. 
§Not  the  same  part  as  Feb.  26. 


198  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

In  the  preceding  table  I  have  given  the  separate  results  of  each  set  on  each  mass. 

The  numbers  in  the  table  are  those  by  which  the  brightness  of  the  standard  part 
of  the  mass  D  must  be  multiplied  to  give  the  brightness  of  the  mass  measured.  The 
sources  of  error  were  two:  First,  all  masses  were  compared  with  a  part  of  D,  and  it 
was  intended  to  compare  them  with  the  same  part  of  D;  this  may  not  have  always  been 
done.  Second,  the  lamp  did  not  maintain  a  constant  brilliancy,  and  in  spite  of  precau- 
tions taken  to  eliminate  the  bad  effects  of  this  change,  errors  still  remain  due  to  this 
cause. 

Taking  the  difference  between  the  greatest  and  least  readings  on  each  mass  for 
each  night  when  more  than  one  determination  was  made,  we  have  the  following  table. 
The  range  here  given  is  in  per  cent,  of  relative  brightness. 

Mean. 

On  the  mass  A ;  range  =  -.07,    10    -     -     -     -  0.09 

F;  .09,  .07,  .09,  .15,  .11,  .13      -  -  o.n 

G;  .08,  .27,  .12,  .14,  .16  -  0.15 

[G] ;  .10,  .11     -     -     -     -     -     -  -  o.i  i 

-  -  o.  1 1 

-  -  o.  i  o 
.08  -  0.13 

-  -  0.05 

-  -  0.04 

-  -  0.13 

Mean     -     -     o.  10 

The  probable  accidental  error  is  thus  much  below  10  per  cent. 

It  should  also  be  noticed  that  for  every  night  where  there  is  more  than  one  deter- 
mination for  the  same  mass,  and  where  the  second  of  such  ratios  is  smaller  than  the 
first,  it  is  the  same  for  every  other  mass  on  that  night.  This  shows  that  much  of  the 
error  is  in  assuming  the  zero  for  the  lamp,  but  that  the  relative  brightness  of  two  masses 
F  and  G,  for  example,  is  still  to  be  depended  on.  Having  regard  to  what  goes  before, 
I  conclude  that  the  mass  A  is  certainly  brighter  than  D,  though,  probably,  not  very 
much  brighter. 

F  and  G. 

Collecting  the  results  of  these  measures,  we  see  that 

1878,  Jan.  26.  F  >  G 
1878,  Feb.    5.  G  >  F  (probably). 

1878,  Feb.  26.  G>  F  (this  differs  from  the  eye  observation  of  this  date). 
1878,  Mar.  9.  F  >G 
1880,  Jan.  10.  G>  F 
1880,  Jan.  13.  F  >G 
1880,  Jan.  14.  G>F 
1880,  Jan.  1 6.  G>  F 

It  seems  to  me  clear,  after  making  allowance  for  all  sources  of  error,  that  F  and 
G  change  in  relative  brightness,  as  was  suggested  by  eye  observations  alone. 


H; 

.1  I 

- 

-     - 

- 

-     -     - 

I; 

.04,   . 

10, 

.19,  . 

09, 

.10    -       - 

E; 

.07, 

1  4, 

.09,  . 

24, 

.11,   .21, 

Q; 

•05 

- 

-     - 

- 

-       -       - 

f, 

.04 

- 

-     - 

- 

-       -       - 

[D];  -is 

MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  199 

G  and  H. 

1878,  Feb.  26.  G  >  H  (this  agrees  with  eye  estimate). 

1878,  Feb.  28.  G  —  H  (by  eye  estimate). 

1880,  Jan.     3.  G>  H 

1880,  Jan.  14.  G>  H 

From  these  I  conclude  G  to  be  a  brighter  mass  than  H. 

H,  I,  and  E. 

1878,  Feb.    5. —  I<E. 

1878,  Feb.  26.  H  >  I,  H  >  E,  I  >  E  (also  by  eye). 
1878,  Feb.  28.  H  >  I,  H  >  E,  I  >  E. 
1880,  Jan.     3.  H  >  I,  H  =z  E,  I<  E. 
1880,  Jan.  14.  H<I,  H>E,  I  >  E. 

The  observation  of  Jan.  3  gives  H  >  I  and  H  —  E,  therefore  E  >  I,  and  in  fact 
it  was  so  observed.  On  other  occasions  I  was  certainly  brighter  than  E,  whence  it 
is  concluded  that  I  and  E  vary  in  relative  brightness,  as  do  also  H  and  I. 

E. 

\ 

In  general  the  photometer  results  corroborate  my  strong  impression  previously 
and  independently  recorded,  that  E  became  brighter  relative  to  D  in  the  last  years  of 
observation.  D  has  also  grown  fainter  according  to  my  eye  observations,  and  the 
photometric  observations  I  have  made  agree  with  this  conclusion,  since  the  brightness 
of  all  the  masses  relative  to  D  is  greater  in  1880  than  in  1878.  The  conclusions 
which  I  am  disposed  to  draw  from  the  above  observations  are  these: 

A  has  been  throughout  my  observations  the  brightest  mass  of  the  Huyglienian  region. 
E  has  grown  brighter  from  1874  to  1880,  while  D  has  grown  fainter  in  the  same  time. 
The  masses  F,  G,  H,  and  I  change  in  absolute  brightness.  Although  at  first  sight  it 
might  seem  that  such  observations  as  I  have  made  should  have  yielded  definite  numer 
ical  results,  yet,  I  think,  that  a  consideration  of  the  difficulties  and  uncertainties  will 
excuse  me  from  attempting  to  draw  any  more  exact  conclusions  from  this  part  of  the 
work.  I  conceive  that  I  have  demonstrated  the  existence  of  certain  changes  of  bright- 
ness, and  I  am  disposed  to  leave,  for  the  present,  the  question  as  to  the  exact  amount 
of  these,  open. 

It  will  be  noticed  that  the  results  independently  obtained  from  the  eye  observa- 
tions of  1875-1878  and  from  the  photometric  observations  of  1878-1880  agree. 


2OO  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

PART  III. — SUMMARY  OF  ALL  THE  OBSERVATIONS  (1656-1880)  WHICH  HAVE  BEEN  PRE- 
VIOUSLY GIVEN  IN  DETAIL. 

In  what  follows  I  have  collected  all  the  observations  which  have  been  given  in 
detail  in  the  preceding-  pages,  and  have  separated  them  into  divisions.  Each  division 
treats  of  the  observations  which  have  been  made  on  some  separate  mass  or  channel 
or  point,  and  the  various  observations  are  arranged  in  chronological  order  under  each 
head.  In  this  way  it  will  be  easy  to  see  if  there  are  evidences  of  progressive  and  regular 
change  in  any  of  the  masses  considered.  Irregular  changes  can  hardly  be  detected 
in  this  (or  any  other)  way,  since  the  observations  and  drawings  considered  are  the 
work  of  so  many  different  observers  so  variously  equipped  and  in  so  many  different 
stations. 

I  desire  to  call  attention  to  the  way  in  which  the  following  summary  has  been 
made.  The  evidence  presented  is  derived  from  the  recorded  observations  and  from  the 
drawings  of  all  the  observers  since  1826.  Blank  forms  were  prepared,  one  for  each 
mass,  as  A,  E,  etc.,  and  on  the  edge  the  names  of  the  various  observers,  witli  the 
dates  of  observation,  were  written. 

As  the  various  memoirs  came  into  my  hands  I  read  them  carefully  and  entered 
any  important  remarks  under  their  appropriate  headings.  In  this  way  these  sheets 
were  filled  up  in  an  irregular  order  (not  chronologically),  and  my  judgment  was  left 
entirely  unbiassed.  The  drawings  were  treated  in  the  same  way.  The  original 
engravings,  and  in  many  cases  the  original  pencil  drawings,  or  photographs  of  them 
were  consulted,  and  all  the  evidence  I  obtained  from  them  was  entered  in  its  appro- 
priate place.  After  the  sheets  were  full  or  nearly  so  (in  1880)  each  sheet  was  con- 
sidered by  itself  and  my  final  conclusion  reached.  This  conclusion  can  be  tested  by 
any  one  at  a  glance.  The  testing  of  the  summary  itself  will  require  a  reference  to  the 
original  paper,  which  is  not  always  easy.  To  facilitate  this  I  have  in  many  cases 
added  here  a  reference  to  the  page  from  which  the  statement  was  taken.  I  trust  I 
.have  made  it  plain  that  not  only  was  the  summary  made  without  any  preconceived 
notions  of  what  was  to  be  expected,  but  that  such  ideas,  even  if  present,  could  not 
have  been  recorded  in  these  tables,  owing  to  the  way  in  which  they  were  constructed. 

The  final  discussion  of  these  sheets  was  put  off  till  the  last  moment,  when  all 
the  evidence  was  in.  With  regard  to  the  nature  of  the  drawings  themselves  it  may 
not  be  out  of  place  to  say  a  few  words.  I  am  acquainted  with  but  one  drawing  of  the 
nebula  which  is  entirely  above  criticism,  that  of  the  late  Gr.  P.  BOND.  (See  frontis- 
piece.) He  was  himself  a  skilled  artist,  and  he  had  been  familiar  with  the  nebula  for 
fifteen  or  twenty  years.  He  made  scores  of  drawings  in  white  and  black  and  the 
reverse,  in  colors,  etc.  I£ach  of  these  was  revised  and  re-revised  many  times.  The 
final  drawing  in  water-color  was  copied  by  Mr.  WATTS,  a  skillful  engraver,  who  him- 
self was  extremely  familiar  with  the  nebula  from  repeated  views  and  studies  of  it 
through  the  Harvard  refractor. 

The  revisions  of  the  original  plate  lasted  many  months,  and  I  have  myself  exam- 
ined from  fifteen  to  twenty  final  revises  of  the  plate.  Color,  form,  and  relative  brill- 
iancy were  all  successively  and  exhaustively  criticised,  and  Professor  BOND  expressed 
himself  as  fully  satisfied  with  the  plate  in  every  essential  feature. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  2OI 

Add  to  this,  that  with  the  exception  of  a  few  points,  elsewhere  considered,  this 
engraving  has  been  constantly  satisfactory  to  me  in  my  very  frequent  comparisons  of 
it  with  the  nebula  even  under  the  severest  criticism  which  I  could  apply,  and  it  is  safe 
to  say  that  this  nebula  as  it  appears  in  a  refractor  is  satisfactorily  represented. 

The  drawing  (white  on  black)  of  Lord  ROS.SE  appears  to  deserve  almost  equally 
high  praise,  but  as  I  have  never  seen  the  nebula  through  a  reflector,  and  as  the  pictorial 
effect  of  the  nebula,  as  seen  in  Washington,  is  somewhat  different  from  that  presented 
by  Lord  ROSSE'S  large  engraving,  I  cannot  speak  with  proper  certainty  upon  this  point. 
As  a  map  of  the  nebula  the  drawing  of  Lord  ROSSE  (black  on  white)  is,  as  has  often 
been  said,  almost  perfect,  but  I  presume  that  it  was  not  intended  in  any  way  as 
a  pictorial  representation,  but  rather  as  a  scheme  for  representing  by  conventional 
signs  (as  in  topographical  color-drawing)  the  features  examined.  As  pictorial  repre- 
sentations the  original  pastel  drawings  of  M.  TROUVELOT,  with  the  Harvard  College 
refractor  and  with  that  of  the  Naval  Observatory,  are  extremely  fine,  but,  as  in  these 
cases,  sufficient  time  was  not  available  for  the  study  of  the  various  objects  they  are 
lacking  in  minute  accuracy  of  detail.  Most  other  drawings  of  the  Orion  nebula,  except 
those  mentioned,  fall  into  the  class  of  maps,  which  give  by  conventional  signs  the 
features  examined.  The  nature  of  these  signs  is  not  perfectly  fixed,  and  has  in  each 
case  to  be  determined  from  the  drawing  itself  on  the  supposition  that  some  features, 
at  least,  are  now  as  they  were  at  the^time  of  the  drawing.  . 

Their  examination  is  rendered  more  difficult  by  the  shortcomings  of  the  engraver 
or  of  the  process  of  delineation  adopted,  and  this  is  why  the  examination  of  so  many 
original  drawings  has  been  made.  I  have  expressed  elsewhere  my  thanks  to  the 
numerous  gentlemen  who  have  aided  me  in  obtaining  access  to  the  various  originals, 
and  particularly  to  Miss  CAROLINE  LASSELL,  who  has  made  for  my  use  a  fine  fac  simile 
drawing  of  the  (unpublished)  Malta  drawing  of  1864. 

There  is  only  one  way,  of  which  I  am  aware,  for  avoiding  undue  bias  in  the 
drawing  of  nebulae,  and  that  is  in  making  one  drawing  in  the  ordinary  way  with  an 
inverting  eye-piece,  and  others  through  reflecting  prisms,  or  erecting  eye-pieces  which 
will  so  alter  the  usual  appearances,  without  changing  the  real  brilliancy  and  shape, 
that  an  unbiased  judgment  can  be  made  by  the  artist;  I  should  rather  say,  from  trials 
by  myself,  so  that  it  is  scarcely  possible  for  him  to  have  a  hurtful  bias.  This  method 
I  confidently  commend  to  those  possessed  of  more  artistic  skill  than  myself.  In  my 
own  case  1  am  certain  from  repeated  trials  that  a  definite  amount  of  time  can  be  more 
advantageously  expended  in  measures  and  verbal  descriptions  than  in  sketches,  and 
this  is  why  I  have  not  attempted  to  add  another  drawing  to  the  many  excellent  ones 
already  available,  but  have  confined  myself  to  the  outline  Index-Map.  Even  this  is 
by  no  means  correct  even  as  to  form.  The  results  of  my  work  must  be  looked  for  in 
the  preceding  and  following  summaries  of  Washington  observations. 

LAMONT  (Ucler  die  Nebelflecken,  Munich,    1837,  p.   22)  gives  an  account  of 
obvious  reasons  for  the  different  appearances  of  the  same  nebula  in  various  telescopes, 
which  is  at  the  same  time  so  brief  and  so  clear  that  I  quote  it  in  full :  ' 
eine  Flache  vor,  die  aus  kleinen  Abtheilungen  von  verschiedenen  Lu-htstarke  1 
durch  serimre  Zwischenriiume  von  einander  o-etrennt,  so  werden  die  schwacheren 
Abtheilungen  in  kleineren  Fernrohren  unsichtbar  bleiben:  die  Starkeren  a 
APP.  V 20 


202  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

sie  nalie  an  einander  liegen,  durch  Verschwinden  der  Zwischenniume  in  erne  gleich- 
formig  erleuchtete  Fliiche  iibergehen.  Diess  1st  nun  gerade  der  Erfolg,  den  die  Beo- 
bachtung  an  den  Nebeln  nacliweist.  Ein  schwaches  Fernrohr  zeigt  den  mittlereii  Theil 
des  Orion-Nebels  gleichformig  erlenchtet,  ein  stiirkeres  giebt  ihm  ein  flockenartiges 
Ansehen,  und  der  hiesige  Refractor  lost  ihm  in  einzelne  messbare  Abtheilungen  auf, 
wie  in  Fig.  XI  (our  Fig.  20)  zeigt.  Eben  so  muss  ein  Nebel,  der  gegen  die  Grenze 
immer  schwaeher  wird  bis  er  sich  ganzlich  verliert,  in  einem  grossen  Fernrohre  viel 
ausgedelmter  erscheinen,  als  in  einem  kleineren  Fernrohre,  Avelches  dieselbe  vergros- 
serung  hat." 

The  evidence  to  be  obtained  from  all  the  drawings  of  the  nebula  is  presented  and 
discussed  in  the  tables  that  follow.  If  two  or  more  drawings  are  made  cotempora- 
neously,  the  points  in  which  they  agree  may  be  taken  to  be  correct.  Points  of  differ- 
ence must  be  examined  in  the  light  of  the  conditions  under  which  the  drawings  are 
made.  The  presumption  is  always  against  differences  of  delineation  corresponding  to 
real  differences  in  the  surfaces  and  features  drawn. 

With  this  preface  I  proceed  to  the  detailed  consideration  of  the  history  of  each 
particular  mass.  This  is  contained  in  the  pages  immediately  following,  where  the 
masses  are  arranged  in  order  of  right  ascension  (nearly). 

Reference  should  be  made  to  the  Index-Chart  and  to  my  detailed  observations  in 
doubtful  points. 

Connection  between  the  nebula  of  Orion  and  its  contained  stars. 

A  physical  connection  between  the  nebula  of  Orion  and  its  contained  stars  is 
suggested  by  several  circumstances.  One  of  the  most  striking  indications  is  found  in 
a  comparison  between  the  number  of  stars  in  the  same  area  in  different  parts  of  the 
nebula,  with  similar  instruments,  such  as  the  Pulkova  and  Harvard  College  refractors, 
for  example.*  If  the  stars  observed  by  BOND  in  the  whole  nebula  had  been  uniformly 
distributed  through  it,  we  should  have  expected  to  find  about  72  in  the  central  portion 
worked  over  by  STRUVE;  as  a  matter  of  fact  STRUVE  has  155  (and  BOND  even  more), 
which  alone  constitutes  a  strong  argument  for  a  connection  between  brightness  in  the 
nebulous  matter  and  number  of  stars.  As  has  been  pointed  out  (V.  J  S.  der  Ast.  Gesell, 
1868,  p.  31),  it  is  necessary  to  this  argument  that  BOND  should  have  given  equal  atten- 
tion to  all  parts  of  the  nebula.  Accurate  information  as  to  this  point  is  not  attainable, 
but  from  what  is  known  of  BOND'S  method  of  working,  and  of  the  exceeding  patience 
and  minuteness  with  which  this  research  was  prosecuted,  there  can  be  no  doubt  that 
much  force  attaches  to  this  consideration. 

Order  of  brightness  of  the  various  masses. 

This  order  has  usually  been  derived  from  the  drawings  published,  but  sometimes 
from  original  pencil  drawings  or  from  the  notes  of  observation.  The  order  of  bright- 
ness is  usually  given  in  three  grades — i,  2,  3  ;  i  being  the  brightest.  The  interval 
between  these  grades  is  by  no  means  equal.  Where  a  letter  is  inclosed  in  a  paren- 
thesis, as  (A),  only  a  part  of  the  designated  mass  is  referred  to.  The  letters  are 
arranged  under  each  head  in  the  order  of  brightness,  so  far  as  it  can" be  made  out. 

*  See  also  Lord  ROSSE  on  this  subject  in  the  extracts  from  his  memoir  previously  given,  p.  87. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  203 

HERSCHEL,  1824  (published  engraving).  There  are  no  separate  masses  laid  down,  and 
the  comparison  is  somewhat  doubtful  from  this  cause.  It  appears  to  be,  i.  E,  F, 
G,  H,  N,  M,  0,  Q,  D.  2.  A,  g,  B,  W.  The  last  not  much  brighter  than  <7. 

HEESCHEL,  1837  (published  engraving),  i.  E,  I,  G,  N,  Q.  2.  F,  H,  S,  M,  R,  P  (all 
equally  bright).  3.  D  =  <7  (this  is  very  different  from  other  drawings).  4.  A. 

LAMONT,  1837  (published  engraving),  i.  (W)  I,  E,  (A),  (A),  G,  F,  (Q).  2.  D,  S,  or 
R,  e  =  Z,  K  (W) ;  part  of  A?  E  and  I  brighter  than  all  the  rest,  D  —  Q. 

LAMONT,   1837  (from  an  original  pencil-drawing  kindly  lent  me  by  Dr.  DOBERCK). 

1.  I,  E,  F,  G,  A,  Q,  D,  H.     2.  C,  B,  W,  J,  and  all  the  rest  about  equal. 

DE  Vico,  1839  (published  engraving).  No  separate  masses;  and  therefore  the  order 
here  set  down  is  somewhat  doubtful,  i.  D,  W,  A,  h.  2.  Q,  H,  G,  F,  E,  I. 

LASSELL,  1847  (engraving),  i.  E  ?  -  -  I,  D,  (<r)  F,  G;  the  south  shore  of  G  is  the  part 
referred  to.  2.  Q,  W,  Q,  P,  S,  A,  J,  and  A  =  ju  3.  B,  etc. 

W.  C.  BOND,  1848  (from  published  engraving),     i.  (J),  (I)  (preceding  parts  of  these). 

2.  (G),  (N),  (F),  (D).     3.  The  following  part  of  A  quite  faint;  as  faint  as  G. 
W.  C.  BOND,  1848  (from  original  note-books  and  drawings  kindly  put  at  my  disposi- 
tion by  the  director  of  the  Harvard  College  Observatory,  Prof.  E.  C.  PICKERING). 
i.  D,  B,  A,  and  G,  H.     2.  E  and  F,  I;  again  in  other  places  D;  J?,  I,  G,  H, 
F  ?,  Q,  N,  A.     See  observations. 

LIAPONOFF,  1851  (published  engraving),  i.  A.  B.  2.  D1,  F1,  0,  I;  all  these  not  very 
different.  3.  G  (p.  75)  H,  E  (from  text  of  memoir).  In  general  the  5.  half  of  the 
Huyghenian  region  is  brighter  than  the  n.  half  (p.  82).  D>  F,  but  not  much; 
Q  —  D  (p.  77)  ;  I  —  H  (p.  75).  G>H>EandI>E;A  very  bright,  and 
W  very  faint  (p.  75).  SECCHI  says  that  LIAPONOFF  gives  B  F  the  maximum  light. 

LASSELL,  1854.  Some  copies  of  the  published  engraving  are  poor  on  account  of  the 
plate  having  worn,  but  I  have  made  these  estimates  from  a  beautiful  fresh  copy 
kindly  given  me  by  Mr.  LASSELL.  i.  F,  G,  I.  2.  D,  g  (B!).  3.  H,  N  (Of). 
A,  totally  wanting. 

SCHMIDT,  1861  (from  original  drawing  kindly  given  to  me  by  Dr.  SCHMIDT).  R,  Q,  I, 
L,  G,  H,  E,  F,  A  and  B  faint. 

LASSELL,  1862  (a  careful  copy  of  his  original  drawing),  i.  A,  B,  D.  2.  W,  I,  G, 
R,  (P),  E,  and  Y.  Perhaps  this  is  not  very  certain,  although  I  had  the  advantage 
of  a  copy  of  Mr.  LASSELL'S  original  drawing  made  by  Miss  LASSELL. 

G.  P.  BOND,  1865  (published  engraving),  i.  A.  2.  D,  I.  3.  F,  G,  H,  E.  All 
of  classes  2  and  3  nearly  equal. 

G.  P.  BOND,  1863.  Completed  drawing  based  on  many  studies,  and  "compared  with 
the  heavens  about  Feb.,  1863."  i.  A,  part  of  B!  2.  D,  I,  Q,  F,  G,  E,  H  (<r). 
H  is  next  to  A  in  brightness,  determined  photometrically  by  the  order  in  which 
the  masses  appeared  as  twilight  disappeared.  It  certainly  is  not  so  now.  Note  of 
1859,  March  23. 

WEBB,  1866  (from  original  drawing,  etc.,  kindly  lent  me  by  Dr.  WEBB). 
feeble."     D  >  A.     E  nearly  equal  to  to  G,  F,  and  I. 

ROSSE,  1867.  See  extracts  from  the  memoir  ante,  where  the  order  of  brightness  is  care- 
fully given.  From  the  drawing  it  is  about  as  follows:  N,  G,  F,  I,  E,  A,  (B),  Q,  N, 
P,  etc. 


2O4 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


SECCHI,  1868.  i.  E,  I,  F,  H,  N,  D.  2.  (W),  (B),  and?  A.  E  >  G,  F.  B  and  J  < 
north  shore  of  K.  The  line  of  maximum  light  is  G  F  of  LIAPONOFF  (that  is  [G] 
[F]  of  our  Index-Chart).  The  description  by  SECCHI  gives  the  following  results: 
E>GandF;  GandH>M?  andS?  SECCHI'S  a — Washington  F;  b  —  G;  c=H; 
d  zz  I.  G  is  one  of  the  brightest  masses  (p.  1 4) ;  [D]  >  E ;  D,  E  , B  >  K.  Accord- 
ing to  LIAPONOFF  the  line  of  maximum  brightness  is  along  the  line  647  to  point  of 
Q;  according  to  SECCHI  from  647  to  W1.  W?  and  D?  are  the  brightest  masses. 

D'ARREST,  1 872  (published  engraving),    i.  W,D,Q,P,R,O.     2.  J,  I,E,  (Hand  G),  F. 

3.  A  —  brightest  part  of  IJL. 

WINLOCK  and  TROUVELOT,  1874  (published  engravingX     i.  A.     2.   D,  (<y),  (v).     3.  Q. 

4.  J,  G.     5.  H,  F.     6.  I,  E. 

TEMPEL,  1876  (from  a  photograph  kindly  sent  me  by  M.  TEMPEL).  All  the  principal 
masses  about  equal  in  light,  and  G  about  the  same  brightness. 

TROUVELOT,  1876?  Reference  is  made  to  M.  TROUVELOT'S  observations,  ante,  p.  105. 

HOLDEN,  1877.  For  a  resume  of  the  conclusions  to  be  derived  from  the  Washington 
Observations  reference  is  made  to  the  pages  where  they  are  given  in  detail.  The 
conclusions  there  given  are  corroborated  by  the  facts  summarized  here. 

LANGLEY,  1879.  Reference  is  made  to  Professor  LANGLEY'S  observations  and  Fig.  39. 

Frons. 


Observer. 

Date. 

P 

Remarks. 

HERSCHEL    .     .     . 

1826 

43-0 

Convex  to  685. 

HERSCHEL    . 

1837 

56.0 

Concave  to  685. 

LAMONT  .... 
DE  Vico 
LASSELL  .... 

1837 
1839 
1847 

45-0 
47  ± 

Only  of  that  part  between  E  and  the  R.  A.  of  685.  [Concave  towards 
685.]  N.  B.  —  Those  remarks  under  LAMONT  which  are  inclosed  in  [] 
refer  throughout  these  tables  to  his  original  pencil  sketch,  kindly  com- 
municated by  Dr.  DOBERCK. 

W.  C.  BOND      .     . 

1848 

49-7 

LIAPONOFF  .     . 

1851 

37-0 

LASSELL  .... 

1854 

60.0 

SCHMIDT 

1861 

52.1 

LASSELL  .... 

1862 

52.7 

G.  P.  BOND  .     .     . 

1865 

47-2 

A  little  brighter  along  the  edge  than  within  this  line.  The  brighter  strip 
is  about  10"  wide. 

ROSSE     .... 

1867 

49-5 

Convex  to  685. 

SECCHI    .... 

1868 

42.0 

D'ARREST      . 

1872 

5r.o 

WINLOCK     .     .     i 
TROUVELOT  .     .     } 

1874 

53-5 

TEMPEL  .... 
HOLDEN  .... 

1876 
1877 

49.0? 
50.3 

The  original  photograph  is  on  too  small  a  scale  to  permit  of  accurate 
measures  on  the  enlarged  copy,  especially  of  measures  of  Aa  and  Atf. 
The  frons  is  convex  towards  685. 

MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

There  is  no  evidence  of  any  change  since  1 826  in  the  frons.  The  error  of  making 
the  frons  concave  towards  685  is  easily  committed,  as  Lord  ROSE  pointed  out  to  me 
in  1875  in  a  letter. 

I  must,  however,  point  out  the  singular  shape  of  the  apex  (E)  of  the  Huyghenian 
region  during  the  period  1 771-1800.  LE  GENTIL  (i  758)  shows  the  angle  of /row*  and 
occiput  obtuse.  MESSIER  (1771)  gives  a  prolongation  of  E  southwards.  SCHROETER 
(1794)  has  the  same  horn-like  protuberance  towards  the  south  in  all  his  drawings. 
He  was  familiar  with  MESSIER'S  work,  and  in  his  first  drawing  ( 1 794)  may  have  copied 
this  feature  from  MESSIER,  though  it  is  not  likely.  In  his  drawing  of  Feb.,  1800,  which 
is  probably  independent  of  the  other,  the  same  figure  is  repeated.  From  HERSCHEL 
(1824)  to  the  present  time  there  can  have  been  no  material  change.  I  regard,  then, 
this  feature  as  constant. 

Occiput. 


Observer. 

Date.              p 

Remarks. 

MESSIER. 

1771           142.5? 

HERSCHEL    .     . 

1826           133.0 

HERSCHEL  remarks  that  it  is  perpendicular  to  the  frons. 

HERSCHEL    . 

1837           142.5 

\ 

LAMONT  .... 
DE  Vico      .     .     . 

1837           134.0 
1839           130.0 

[Parallel  to  the  preceding  edge  of  F,  and  outline  better  defined  and  more 
sharp  than  that  of  the/Vw/j.] 

RONDONI 

1841             .     . 

LASSKLL  .... 

1847 

W.  C.  BOND      .     . 

LlAPONOFF    .       . 

1848           147-5 
1851           137-0 

1847,  Dec.  6  (Harvard  College  Obs.    Note  Book).  "  575  is  just  on  the  edge 
of  the  nebula." 

LASSELL.  . 

1854           140.0?     . 

SCHMIDT      .     .     . 

1861            143-0? 

Only  of  that  part  south  of  J. 

LASSELL. 

1862           139-° 

Exclusive  of  Y. 

G.  P.  BOND  .     .     . 
ROSSE     .... 
SECCHI    .... 

1865           136.0 
1867           136.0 
1868           138.5 

G.  P,  BOND  says  (1864,  Jan.  17)  that  "  575  is  just  on  the  edge  of  the  nebula 
with  an   uncertainty  of  i"".     1864,  Feb.  12,  "It  is  critically  placed  on 
the  preceding  edge  of  the  bright  edge  of  strong  light  [B]. 
Angle  at  E  of  frons  and  occiput  "  almost  90°  "  (text,  p.  13). 

D'ARREST      . 

1872           142.1 

WlNLOCK       .       .       ) 

TROUVELOT  .     .      > 

1874           142.0 

TEMPEL  .... 
HOLDEN  .... 

1876           134-0? 
1877           141-1 

p  _  I;}6°  i  Of  the  occiput  exclusive  of  Y  (see  LASSEI.L,  1862). 

Probably  constant. 

It  should  be  remarked  that  the  angle  between  the  frons  and  orciput  is   116°, 
according  to  HUYGHENS  (1694);    129°,  according  to  LE  GENTIL 
present  time  (1824  to  1880). 


2O6 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


./'arallel  through  0'  (628)  towards  the  east. 

The  numbers  in  the  column  4a  indicate  the  length  of  that  portion  of  the  above 
line  which  is  involved  in  nebulosity.  Only  so  much  of  the  line  is  considered  as  lies 
between  628  and  708.  This  line  is  about  150"  in  length. 


Observer. 

Date. 

M 

Remarks. 

HERSCHEL    . 

1826 

all 

It  is  all  involved  in  the  bright  nebulosity. 

HERSCHEL    . 

1^37 

all 

It  is  all  involved  in  the  bright  nebulosity. 

LAMONT  .... 

1837 

[4i"l 

[About  three-elevenths  of  the  distance.] 

DE  Vico 

1839 

50": 

About  one-half  involved. 

LASSELL  .... 

1847 

? 

Only  a  portion  involved. 

W.  C.  BOND      .     . 

1848 

150" 

All  involved. 

LlAPONOFF    . 

1851 

60" 

LASSELL  .... 

,•1854 

? 

SCHMIDT.     .     .     . 

1861 

all 

LASSELL  .... 

1862 

all 

G.  P.  BOND  .     .     . 

1865 

37".  5 

I 

ROSSE     .... 

1867 

42" 

SECCHI    .... 

1868 

150" 

All  involved. 

D'  ARREST    . 

1872 

100" 

WlNLOCK       .       .       1 

TROUVELOT  .     .     > 

1874 

150"  ? 

All  involved? 

TEMPEL  .     . 

1876 

50± 

Only  about  one-third  of  the  line  628-708. 

1878 

Set  wire  on  parallel  through  ff  =  628.     From  628  to  go  this 

wire   is  im- 

mersed  in  nebula  (77")-     From  628  to  the  R.  A.  of  651  (29"] 

this  line  is 

in  a  darker  space.     It  is  only  a  little  south  of  the  south  border  of  r.     It 

is  immersed  in  R  ;   following  R  it  passes  through  a  dark 

space,  and 

finally  is  involved  in  /*, 

North  point  of  Sinus  Gentilii. 

Observer. 

Date. 

A 

Remarks. 

HERSCHEL  .     .     . 

1826 

122.  6 

HERSCHEL   .     .     . 

1837 

87.5 

LAMONT  .... 

1837 

.       - 

[Not  laid  down.] 

DE  Vico.     .     .     . 

1839 

55-8? 

LASSELL  .... 

1847 

-     - 

W.  C.  BOND.     .     . 

1848 

77.  c 

LlAPONOFF    .       .       . 

1851 

/  .  '     J 

58? 

SCHMIDT 

1861 

40.8? 

LASSELL  .... 

1862 

109.4 

WEBB      .... 

1863 

See  text,  p.  78,  et  seq. 

G.  P.  BOND  .     .     . 

1865 

57-5 

On  original  drawing  from  which  engraving  was  made,  a  few  seconds 

south  of  this. 

WEBB     

1866 

In  same  Ad  as  south  point  of  G. 

ROSSE     .     .     . 

1867 

70 

SECCHI    .     .     .     . 

1868 

97-3 

D'ARREST     .       .       . 

1872 

60 

WlNLOCK       .       .      ) 

TROUVELOT  .     .     ) 

1874 

77-5 

H  OLDEN.       .       - 

1877 

66.7 

Probably  constant. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

C. 


2O7 


With  regard  to  c  it  should  be  remarked  that  D'ARREST  makes  it  concave  towards 
the  north.     This  is  different  from  all  other  authorities. 

Z. 

ROSSK,  1867,  JS  of  north  point—  120". 
SECCHI,  1868,  JS  of  north  point  —  140". 

d. 

BOND,  1865,  J6  of  north  point—  105",  Ja  =  —  6o". 
ROSSE,  1867,  J6  of  north  point  — 94". 

D'ARRE&T,  1872,  Jd  of  north  point  —  134".     (Very  probably  this  is  a  part  of  Z, 
seen  differently  from  the  other  drawings). 

HOLDEN,  1877,  Jd  of  north  point—  ioo".g. 
Probably  constant. 

L. 

W.  C.  BOND,  1848,  Jd  north  point  =  o"  ?  ? 

ROSSE,  1867,  J6  north  point  —  ^o"  ? 

HOLDEN,  1877,  ^  north  point—  25". 5. 

It  is  difficult  to  say  if  this  corresponds  to  any  real  difference  ;  most  probably  not. 

Angle  of  preceding  edges  of  J  and  B. 


Observer. 

Date. 

P 

0 

HERSCHEI.   .     - 

1826 

.     . 

HERSCHEL    . 

1837 

LAMONT.     .     .     . 

i837 

[24°] 

DE  Vico.     .     .     .        '1839 

174? 

LASSELL  .... 

;847 

W.  C.  BOND.     .     . 

1848              39? 

LIAPONOFF  .     .     .  |       1851               10.2 

LASSELI  

1854 

-      - 

SCHMIDT     .     .     - 

1861 

172.5 

LASSELL  .... 

1862 

i8o± 

G.  P.  BOND  .     .     - 

1865 

6.1 

ROSSE     .... 

1867 

O.  2 

SECCHI    ....         1868             13 

D'ARREST     .      -      .            1872                 165 

WISLOCK                  \        1874                7-2 

TROUVELOT  .     .      > 

TEMPKI  1876             30?? 

HOLDKN.     .     -     -         1877                7-7 

Remarks. 


[J  tolerably  bright.]  [  8  does  not  point  towards  575,  but  to  the  west  of  it.  At 
least  the  maximum  brightness  (20"  to  the  north  of  575)  is  on  a  meridian 
halfway  between  575  and  589.] 

For  position  of  575  see  "Occiput". 

B  forms  with  the  following  edge  of  A  and  part  of  I  the  hcmicyclnim  Liafomn-h. 

The  south  part  of  J  and  the  north  part  of  I  have  sonu'tuncs  appeared  to  L. 

"  perfectly  black,"  text  p.  78. 
These  masses  are  further  towards  the  east  than  in  other  drawings. 

"1864,  Apr.  16.    BOND  says,    'This   edge  of    nebula  in    meridian  precisely 
through  575.'"     This  is  the  angle  of  the  preceding  edges  of  the  entire  two 
masses.     For  position  of  575  see  "Ocdput". 
Do. 

This  includes  only  that  part  south  of  0'. 

The  shape  of  these  edgesjiuite  different  from  those  in  any  mddern  figure. 

This  refers  only  to  the  part  south  oft)'. 


208 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


Probably  constant  in  position.     There  may  have  been  changes  in  brightness  of 
B  and  J.     See  B  following. 

B. 


Observer. 

G.  P.  BOND.  . 
ROSSE  .  .  . 

Date. 

/ 

Remarks. 

1865 
1867 

« 

-     - 

About  as  seen  in  the  heavens  by  me  (1874-'  79). 

B  extends  one-third  of  its  length  beyond  (.>-.)  of  [575].     Quite  different  from 
BOND,  1861. 

SECCHI  .  .  . 
D'  ARREST  .  . 
WINLOCK  .  . 
TROUVELOT  .  . 

1868 
1872 

\        1874 

-     - 

B  begins  to  curve  to  the  west  immediately  north  of  the  parallel  of  6'. 
Quite  different  from  other  drawings;  see  his  figure. 

Different  in  form  and  brightness  from  other  drawings.     Consult  the 

figure. 

HOLDEN  .  . 

1877 

-     - 

About  as  in  BOND'S  drawings,  published  and  unpublished. 

The  brightness  in  Lord  ROSSE'S  drawing  is  different  from  that  in  preceding  and 
following  ones. 

W,  W1,  W2,  W3,  W4,  W5. 


W'. 

Observer. 

Remarks. 

SCHROETER  . 

1794 

W1  =  lacus  Seccliii 

discovered  by  SCHROETER. 

HERSCHEI,   .     .     . 

1824 

W  '  not  laid  down  ; 

W  =  I,  E,  etc.,  in  brightness. 

HERSCHEI,   .     .     . 

1837 

W1  not  laid  down  ; 

W  =  I  in  brightness. 

LAMONT  .... 

1837 

\V  '  not  laid  down  ; 

W  =  T  in  brightness. 

DE  Vico      .     .     . 

1839 

W  not  laid  down; 

W  almost  as  bright  as  any  mass. 

RON  DON  i 
LASSELL  .... 

1841 

1847 

W1  not  laid  down  ; 

\V  almost  missing. 

W.  C.  BOND     .     . 

1848 

W  not  laid  down. 

G.  P.  BOND  ... 

1865 

—  45        +59        Center  of  W1. 

ROSSE     .... 
SECCHI    .... 

1867 
1868 

.      .        +68        A  '5  of  center  of  W 
(brighter  than)  ; 
—  62        +82        Center  of  W1. 

';    diameter  in  (5  24";  W1  and  W-  are  black;   W1  >  r 

w  X;  v>w. 

1 

D'ARREST     .       .       . 

1872 

\V  not  laid  down  ; 

W  =  D  in  brightness. 

WINLOCK     .     .     } 
TROUVELOT  .     .      ) 

•«74 

_5o        -|-74        Center  of  W;   W1 

>  -  ;   c  >  W  ;  W1  >  V,  /.  ,-.,  different  from  1867. 

HOLDEN  .... 

I877 

-  45        +  68 

Probably  constant. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

A. 


209 


Observer. 

Date. 

A« 

HERSCHEL  . 

1826 

// 

HKRSCHEL  .     .     . 

1837 

-     - 

LAMONT  .... 

1837 

-     - 

De  Vico  .... 

1839 

. 

LASSELL  .... 
W.  C.  BOND      .     . 

1847 
1848 

-     - 

LlAPONOFF    . 

LASSELL  .... 

1847 
1854 

-     - 

SCHMIDT      .     . 

1861 

. 

LASSELL       .     .     . 

1862 

. 

WEBK      .... 
G.  P.  BOND  .     .     . 

1863 

1865 

. 

WEBB      .... 

1866 

. 

ROSSE     .... 
SECCHI    .... 

1867 
1868 

3  to  4 

D'ARREST    .     .     . 
WINLOCK     .     . 
TROUVELOT.     .      \ 

1872 
1874 

* 

TEMPEI  
HOLDEN  .... 

1876 
1879 

;; 

Remarks. 


31.6 


A  is  not  laid  down  as  a  separate  mass,  but  it  is  of  the  second  order  of 
brightness ;  by  no  means  one  of  the  brighter  parts. 

A ;  the  following  portion  is  very  faint.  There  is  no  bright  nebulosity  pre- 
ceding the  star  619,  except  a  portion  of  I. 

[The  axis  of  the  brightest  parts  of  A  begins  near  star  608  (not  laid  down), 
and  is  further  towards  the  west  than  now,  though  generally  similar  in 
shape.]  622  is  on  its  following  edge.  A  <  I,  E,  F,  G. 

There  is  no  separate  mass  laid  down,  but  the  space  preceding  0>  is  of  the 

first  order  of  brightness. 
There  is  no  brighter  mass  A. 

The  bright  following  part  of  A  is  not  laid  down  at  all.  On  the  contrary, 
this  portion  is  very  faint.  From  a  MS.  drawing  (no  date)  I  find  both  D 
and  E  brighter  than  A. 

Very  bright ;  text,  p.  77. 

A  is  totally  wanting,  and  its  absence  and  that  of  the  adjacent  parts  is  one 
of  the  most  striking  features  of  thisdrawing. 

A  is  rather  of  the  second  order  of  brightness ;  A  =  Q,  etc. 

A  full  of  singular  convolutions  and  very  bright. 

A  is  by  no  means  the  brightest  mass.     A  <  G  or  F. 

The  brightest  mass,  according  to  a  sketch  1864,  April  16.     A  terminates 

pretty  exactly  at  608. 

Same  as  WEBB,  1863. 

South  point.     A  is  much  as  in  G.  P.  BOND'S  drawing. 

Not  so  bright  as  other  masses. 

Probably  not  so  bright  as  other  masses. 

A  and  the  base  of  B  the  brightest  parts. 

A,  }T\S\.  preceding  the  trapezium  (U  ?),  is  perhaps  the  brightest  mass. 

A  is  the  brightest  mass.    Up  to  1839  the  part  A  is  not  conspicuously  bright. 


In  i847~'48,  two  authorities  make  A  faint,  the  other  very, bright  In  1854  ir  is 
totally  wanting-  (LASSELL),  while  the  same  authority  in  1862  makes  it  the  hrighn-sr 
mass,  and  later  authorities  in  the  main  confirm  this.  It  certainly  seems  to  me  that  A 
about  i847~'48,  and  up  to  1861  was  not  of  its  present  brilliancy.  Compare  LASM-:LL 
1847,  J854,  and  1862.  LIAPONOFF  (1847)  is  opposed  to 'this,  and  his  observations  an- 
entitled  to  the  greatest  weight. 

In  spite  of  this  I  can  reconcile  the  observations  in  no  way  but  by  supposing  sonic 
change  of  brilliancy,  and  I  believe  any  careful  examination  of  the  originals  will  lead 
to  the  same  conclusion. 
App.  V 27 


2IO 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


E. 


Observer. 

Of  line  join- 
ing 628  and 
south  point. 
Date. 

South  point. 

North  point. 

Diam. 
in  dec. 

Remarks. 

/ 

J« 

46 

Ja 

M 

."    0        / 

// 

a 

n 

n 

// 

It  has  a  nucleus  (c)  which  SCHROETER  calls 

SCHROETER 

i  794           -     - 

a  star. 

HERSCHEI.  .     .      1826 

. 

—  10.2 

i7t   2 

Not  a  separate  mass. 

*/j'  * 

HERSCHEL  .     . 

1837 

. 

+  15-° 

140.0 

-      . 

92.5 

46.  5       Uniformly  bright  oval. 

LAMOMT 

1837 

1  80    27 

precedes 

152.6 

•      - 

119-9       32-7 

[E  is  inferior  in  brightness  only  to  I.      Its 
center  is  brighter  than  its  borders.     It 

is  more  sharply  terminated  on  the  side 

towards  I  than  towards  F.] 

DE  Vico     .     . 

1839 

T$Z,1  T 

-        ' 

0.0 

161  7 

Not  a  separate  mass. 

1UO'  / 

RON  DON  i    . 

1041 

T<i<-7 

LASSELL 
W.  C.  BOND    . 

1547 
1848 

-        ' 

*q.  o 

*i6i 

•     • 

-     - 

-      - 

Quite  different  from  all  other  figures.    The 
curious  channel  in  E  does  not  exist  at 

present.     *See  his  memoir,  p.  94. 

LlAPONOFF 

1851 

181  49 

-    4-4 

138  o 

"E  is  considerably  fainter  than  G,  H,  F, 

rCl-  A 

etc."     Text,  p.  75. 

LASSELL 
SCHMIDT    .     . 

I8S4 

1861 

0.0 

127.8 

... 

. 

. 

Rec  angular. 

Drecedes 

127   7 

The  masses    E    and   Y  appear  as  one  of 

LASSELL     .     .      1002 

»*/•  / 

uniform  brightness. 

\X7                                                     1       .  i.'i     , 

Apex  precedes  6'.     E  <  F,  G,  I. 

WEBB    . 
G.  P.  BOND     . 

10UJ 

1865 

-  1-3? 

140.0 

. 

105 

35-  ° 

• 

WEBB    .     .     . 

1866 

. 

0.0 

. 

. 

. 

. 

E  about  the  same  brightness  as  F,  G,  I, 

E<D. 

ROSSE   .     .     . 

1867 

. 

+3  to  4 

135-° 

. 

99 

36.0 

SECCHI  .     .     . 

1868 

. 

+   4-7 

i87±4 

-      - 

140.4 

47 

Distinctly  divided  into  three  oval  masses. 

D'  ARREST  .     . 

1872 

-     • 

+   4 

146 

-     - 

(?) 

-     - 

\VlNLOCK    .        j 

1874 

—  10.  o 

i55-o 

105 

5° 

TROUVELOT      ( 

TEMPEL      .     . 

1876 

-     - 

Scale  too  uncertain  ;  E  has  much  the  same 
shape  as  in  the  Naval  Obser'y  drawing. 

HOLDEN     .     . 

1876 

-     - 

-     0.  I 

144.6 

loo.  3       44.  3 

Probably  constant  in  form. 

It  is  possibly  fainter  now  than  1837. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Dark  channel  between  I  and  [X  and  6r]. 

[The  straight  portion  of  this  is  alone  referred  to.] 


21  I 


Observer. 

Date. 

/ 

Remarks. 

o 

HERSCIIEL   . 

1837 

(  180  > 
(    90  > 

The  south  half;  then  it  turns  at  right  angles. 

LAMONT 
LAMONT       .     .     . 

i*37 
1837 

[44] 

[Inclined  to  fro  as  ;  X  not  laid  down.     The  preceding  end  of  this  channel  is 
perpendicular  to  occiput.  ] 

LASSEI.I  

1854 

60? 

-     ' 

LASSELI  1862 

53? 

Parallel  to  from  '' 

G.  P.  BOND  .     .     . 

1865 

29? 

On  original  drawing  from  which  engraving  was  made  /  seems  rather  larger 
than  this. 

ROOSE     .... 

1867 

28 

SECCHI    .... 

1868 

90 

D'ARREST     .       .       . 

1872 

57 

The  west  portion  only. 

WlNLOCK       .       .         5 

TROUVELOT.     .      > 

1874 

42.5?? 

HOLDEN.       -       -       - 

1877 

25.6 

\                                                       .  __  —  — 

Constant  since  1865,  and  probably  the  earlier  observations  differ  only  on  account 
of  errors.     But  compare  HERSCHEL  (1837). 

Dark  channel  letwcen  E  and  F. 


Observer. 

Date. 

/ 

Remarks. 

' 

o 

HERSCHEL   .     .               '837 
LAMONT       .     -              J837 

160?? 
134?? 

[F  alone  is  sharply  terminated;   E  fades  off  towards  F.] 

W.  C.  BOND      .     . 
SCHMIDT 
LASSEI.I  
WEBB     .     .     -     - 

1848 
1861 
1862 
1863 

143  ?         About  parallel  to  occiput. 

130? 
Neaily  perpendicular  to  from  :    it  is  best  terminated  on  the  following  >idc; 

G.  P.  BOND  .     .     - 
WEBB     .... 

1865 
1866 

132.5? 

Its  prolongation   passes  nearly  through  575  on  original  drawing  from  which 
engraving  was  made. 
Same  as  WEBB,  1863. 

ROSSE     .... 
•  SECCHI  .... 

1867 
1868 

I34-I 
135 

This  channel  is  much  s-horter  than  in  ROSSE. 

D'ARUEST     .       -       - 

1872 

142 

WlNLOCKi      -       -         ) 

TROUVELOT.     .      ) 
TF.MPEL  .     -     - 

1874 
1876 

144-5 
134? 

Parallel  to  occiput. 

1  HOLDEN  .     . 

I                     

1877 

140.9 

Probably  constant. 


212 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

V. 

[The  mark  a  ^>  b  signifies  here  a  brighter  than  l>.~\ 


Observer. 


Date. 


Remarks. 


HERSCHEL  . 
HERSCHEI.  . 
LAMONT .  . 
DE  Vico.  . 
W.  C.  BOND 
LIAPONOFF  . 
LASSELL .  . 
SCHMIDT 
STRUVE  .  . 
LASSELL  . 
WEBB  .  . 
G.  P.  BOND- 
WEBB 

ROSSK     .     . 
SECCHI    .     . 

D'ARREST     - 

WIXLOCK     . 
TROUVELOT. 

HOLDEN  . 


1826 
1827 
1837 

• 

1839 
1848 
1851 

1854 
1861 
1861 
1862 
1863 
1865 
1866 
1867 
1868 
1872 

1874 
1877 


V  is  a  marked  feature ;  see  text.     V  ^>  | ;   V  ^>  south  half  of  T. 

V  is  a  marked  feature ;  it  is  totally  black.     See  text. 

[V  is  not  laid  down  on  his  original  pencil  drawing.]     V  very  much  brighter  than  T. 

V  is  as  bright  as  any  portion. 

V  >  r.     V  <  W.     V  <  Sinus  Genii  Hi. 

"  V  perfectly  black;"  text,  p.  78. 

V  perfectly  black. 

V  ^>  r ;  V  ^>  Sinus  Genii  lit. 

V  is  in  general  quite  dark.     See  (a)  text,  p.  115. 

V  >  r ;  V  =  W1  ?;   V  much  brighter  than  Sinus  Gentilii. 

V  is  not  a  marked  feature. 

V  is  nearly  symmetrically  disposed  about  the  trapezium.   V  ^>  r ;   V  ^>  W;  V  ^>  Sinus  Gent. 

V  better  marked  than  in  WEBB,  1863. 

V  >  r ;  V  =  W1  =  W2. 

V  >  r  ;   V  >  Sinus  Gentilii. 

V  ^>  r,  but  not  much  so. 

V  =  r ;  V  =  Sinus  Gentilii ;  V  <  W1. 
V  <  r1;   V  <  Sinus  Gentilii;  V  <  W1. 


Compare  the  relation  of  V  with  Sinus  Gentilii:  1861-1868,  V>S.  G. ;  1874, 
V  —  S.  G. ;  1877,  V  <C  S.  G.  But  there  has  probably  been  no  marked  change  in  V 
and  S.  G.  Such  observations  are  extremely  difficult,  especially  when  the  area  to  be 
compared  are  not  contiguous.  I  do,  however,  think  a  change  has  taken  place  in  the 
general  brightness  of  T  •  see  under  the  heading  SCHROETEE'S  second  bridge  in  T  in  this 
section. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

F. 


213 


Observer. 

Date. 

South 
point. 

North 
point. 

'  _ 

Brightest  point. 

AS 

AS 

Aa 

AS 

n 

n 

n 

HERSCHEL   .     .     . 

1837 

•     • 

-     - 

-     - 

107.5? 

LAMONT       .     .        !       1837 

08  i  ? 

DEVICO.     .     .     .         1839 

W.  C.  BOND.     .     . 

1848 

LlAPONOFF  . 

LASSELL  .... 

1851 
1854 

-     - 

-     - 

28.6 

97-4 

SCHMIDT 

1861 

. 

68? 

LASSELI  

1862 

WEBB      ....         l8g_ 

G.  P.  BOND  .                    l865 

II7-S        87.5? 

WEBB                               1866 

ROSSE                              toft_ 

• 

ou^ 

114 

. 

. 

m 

SECCHI    .     ...     . 
D'ARREST    .     .     . 

1868 
1872 

144.1 
86 

103.0 

\ 

"5-4 

WlNLOCK     .       .       .   } 

TROUVELOT      .     .  5 

1874 

132.7 

90 

HOLDEN       .     .     . 

1876 

120.  I 

94-9 

+26.8 

102.0 

Remarks. 


Only  the  following  part  laid  down ;  /.  e.,  no  mass  X. 

Middle  point  is  A  d. 
Same  as  HERSCHEL,  1837. 
[F  is  a  right  angle  triangle  in  shape ;  the  hypothenuse 

is  parallel  to  the  line  (612-618)  to  647;  the  base 

lies  in  the /;-<?/«/  the  perpendicular  is  half  as  long 

as  the  base.]     No  mass  X. 
The  brightest  part  is  near  the  frons. 
"Almost  equal  to  D;"  text,  p.  75. 
The  part  of  F  near  X  is  the  brightest. 
The  part  near  X  the  brightest. 
U'est  point  in  A«  =  O".O. 
F  is  best  terminated  o    its  n.  f.  edges. 
F  is  an  equilateral  triangle.     1863,  Jan.  30. 
F  is  oval  in  shape. 

The  south  point  is  in  frons. 

The  triangular  shape  is  like  ROSSE  and  BOND,  but  it 
is  differently  situated.  The  channel  between  F  and 
[G  and  H]  is  different  from  any  other. 

South  point  is  in  the  /tons. 

The  part  of  F  not  near  X  is  the  brightest. 


Probably  constant  in  form.     It  is  possibly  changed  in  brightness,  but  even  this 
by  no  means  proved. 

G. 


IS 


Observer. 

Date. 

Brightest  part. 

Remarks. 

Aa 

Jd 

HERSCHEL  - 

1837 

n 

n 
62.5? 

LAMONT. 

1837 

. 

61.0? 

[G  is  a  circular  bright  mass  of  uniform  brilliancy.] 

W.  C.  BOND      .     . 

1848 

-     - 

-     - 

G  apparently  divided  into  three  parts? 

LlAPONOFF  .       .       . 

1851 

18.5 

63-3 

G  is  fainter  than  the  other  masses  (F,  G,  H);  text,  p.  75. 

LASSELL.     .     .     . 

1854 

-     - 

-     - 

G  is  very  bright  ;  of  the  first  order  of  brightness. 

SCHMIDT 

1861 

-     - 

33? 

G  very  bright;  its  brightest  part  is  certainly  not  so  far  as  JS  =50". 

LASSELL. 

1862 

- 

•     - 

G  very  bright,  and  larger  than  in  BOND,  etc.;  not  square,  but  elongated  in 

WEBB      .... 

1863 

. 

. 

G  is  very  bright,  but  G  <  I.     Its  preceding  edge  precedes  F. 

G.  P.  BOND  .     .     . 

1865 

-     - 

75-  ° 

[The  dark  channel  between  F  and  G  is  situated  just  as  in  1877.) 

WEBB      .... 

1866 

-     - 

-     - 

G  ^>  I.     The  preceding  edges  of  F  and  G  in  same  meridian. 

ROSSE     .... 

1867 

-     - 

72 

i  83"  south  point  :  JS.     [1877,  south  point  :  Jf>  76  ,  HOI.DK.N.] 
(  61"  north  point:   J8. 

SECCHI   .... 

1868         .     . 

. 

G  quite  unlike  BOND  and  ROSSE,  and  present  appearance. 

D'ARREST     . 

1872     ;     .     . 

. 

G  and  H  are  one  mu--. 

WlNLOCK      .       .        ? 

TROUVELOT.     .      ) 

'  1874 

• 

72.5 

TEMPEL  .     .  •  .     . 

1876 

. 

. 

Brightest  point  fnreJcs  628  (a  TV./yV;//). 

HOLDEN.     .     1     . 

1877 

15.9 

65.3 

One  of  the  brightest  masses. 

Probably  no  change  in  form. 


214 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


H. 

ROSSE,  1867,  Jd  south  point  —  83'"! 

ROSSE,  1867,  48  brightest  point—  78"  ? 

HOLUEN,  1877,  46  south  point  —  94".     Probably  no  change. 

D. 


«.  f.  point. 

Observer. 

Date. 

* 

Remarks. 

Ao: 

AS 

HERSCHEL   .     .     - 

1826 

" 

a 
27.7 

II 

74-5 

The  point  of  D  plainly  laid  down.     B  is  concave  towards  the  east. 

SMYTH    .     .     .     -• 

1834 

. 

. 

- 

In  a  coarse  sketch  in  the  celestial  cycle,  ii,  p.  132,  D  is  concave 

to  the  east  and  the  brightest  mass. 

HERSCHEL   .     .     - 

1837 

. 

. 

. 

There  is  no  bright  sharp  following  point,  647  and  651  as  in  Naval 

Observatory  drawing.     D  is  fainter  than  E,  F,  G,  H,  Q,  etc., 

and  concave  to  the  east. 

LAMONT  .... 

1837 

. 

17.4? 

98.1 

647  and  65  1   as  in  Naval  Observatory  drawing.     [651  on  exact 

edge  of  D.     D  has  no  north  sharp  point,  but  its  north  end  is 

rounded.]     [D  brighter  than  H?  but  fainter  than  I.  E,  F,  G, 

(A),  Q,  just  as  in  HERSCHEL,  1837.] 

DfiVico.     .     -     . 

1839 

. 

- 

-     - 

STONEY  .... 

1851 

. 

. 

. 

D  is  partly  concave,  partly  convex  to  the  east,  in  an  unpublished 

drawing  made  at  Lord  ROSSE'S  Observatory. 

W.  C.  BOND  .     .     . 

1848 

. 

16.6 

56.8 

647  and  651  as  in  Naval  Observatory  drawing.     See  text. 

LIAPONOFF  .     .     . 

1851 

, 

26.1* 

55-5* 

A  5  =  69".  6  and  A«  =  26".  5  of  point  of  intersection  of  follow- 

ing edge  of  D  with  north  shore  of  Sinus  magnus.     [*  Co-or- 

dinates of  «s.] 

T    A  CCTTT  T 

181:4 

Following  side  quite  bright. 

.LjAootL..!-.                           -          - 

*^JT- 

1861 

Tangent   to  bright  following  side,   A«  =  i3"-6.     There   is  no 

O  C  I  i  M  IDT            .           -           • 

sharp  northern  point  ;   the  mass  is  oval. 

QTB  TTVT? 

1861 

D  seemed  to  be  subject  to  considerable  variation  both  in  form  and 

o  1  K.  U  V  r.    »        » 

brilliancy. 

LASSELL  .... 

1862 

. 

11.4 

66.1 

641   and  657  in  Naval  Observatory  drawing.     651  as  exactly  on 

edge. 

WFHR 

1863 

D  is  not  so  bright  as  F  ;  no  sharp  north  point  laid  down. 

G.  P.  BOND  ?      .     . 

1865 

? 

36.2 

69.8 

Measured  1864,  April  15,  following  edge  of  D,  A«  (measured) 

=  26".  6  same  date.     647  and  651   as  in  Naval   Observatory 

drawing  except  that  647  is  not  in  a  bay.     "  647  far  within  neb- 

ulosity," 1864,  April  15. 

WEBB     .... 

1866 

D  is  the  brightest  mass  ;  no  sharp  «.  /.  point  laid  down. 

ROSSE     .... 

1867 

-     . 

26 

69 

Same  as  Naval  Observatory  drawing. 

SECCHI    .... 

1868 

. 

44-9 

-  .  - 

A«  of  tangent   to  brightest  of  following  side  of  D.     D  has  no 

bright  sharp  following  point,   and  is  clearly  different  from  its 

present  appearance. 

P'ARREST      . 

1872 

-     - 

32? 

44? 

D  quite  bright,  but  outlines  are  not  so  sharp  as  in  Lord  ROSSE'S 

or  Naval  Observatory  drawing. 

WlNLOCK     .       .       .  1 

1874 

"•3 

92-5 

As  in  Naval  Observatory  drawing. 

TROUVELOT     .     .  > 

HOLDEN  .... 

1877 

. 

7-8 

A«  of  tangent  to  brightest  part  of  following  side  29".  4.     D  is 

convex  towards  the  east. 

The  form  seems  to  have  varied  from  concave  to  the  east  (1826  and  subsequently), 
to  convex  to  the  east  (1865  and  subsequently).     If  the  mass  had  been  always  of  its 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


215 


present  brilliancy,  the  question  of  convexity  or  concavity  could  not  have  taken  a 
moment  to  decide  it.  I,  however,  agree  with  0.  STBUVE  in  believing  this  mass  to  have 
varied  in  brilliancy,  and  I  am  inclined  to  believe  that  the  (possible)  changes  of  form 
are  due  to  this  fact.  The  star  651  seems  to  have  remained  in  a  fixed  position  relative 
to  the  following  edge,  and  no  change  of  form  is  shown  there.  The  extreme  north 
point,  however,  has  certainly  varied  in  brightness  during  the  period  1826-1877;  it 
has  even  so  varied  during  my  own  observations  1874-1880. 

r. 


Observer. 

Date. 

Remarks. 

HERSCHEL   .     .     . 

1826 

North  half:  Streaks  of  nebulosity.     South  half:  Black. 

HERSCHEL   .     .     . 

1837 

r  uniformly  completely  black. 

LAMONT  .... 

1837 

Do. 

DEVico.     .     .     . 

1839 

Do. 

RONDONI 

1841 

LASSEU  

1847 

Notth  half:  Partially  nebulous.     South  half:  Black. 

W.  C.  BOND  .     .     . 

1848 

T  uniformly  completely  black. 

LASSELL  .... 

1854 

North  half  brighter  than  south  half. 

SCHMIDT     ..     -     - 

1861 

Pretty  uniformly  black. 

LASSEI.I  

1862 

Pretty  uniformly  black,  except  for  SCHROETER'S  second  bridge. 

WEBB      .   ".     -     - 

1863 

All  filled  with  nebula. 

G.  P.  BOND  .     .     . 

1865 

The  north  preceding  part  is  brighter  than  the  south  following  part.     Black  streak  near  fvns 
Schroctcrii.     The  south  shore  of  T  reaches  to  the  declination  of  6'  ;  i.  e.,  AS  =  .0". 

ROSSE     .     .     .     - 

1867 

Two  black  streaks  in  it  on  its  west  and  east  sides  ;  the  north  half  brighter  than  south  half. 

WEBB      .... 
SECCHI    .     .     .     - 

1866 
1868 

Completely  filled  with  nebula. 
On  the  whole,  n.  ^  >  s.  *4,t  although  somewhat  doubtful  on  account  of  absence  of  definite 
outline  to  potts  Schroeteni.     The  preceding  edge  is  brighter  than  the  other  portions;  and 
T  extends  to  the  south  of  6'.     T  much  darker  in  1867  than  in  1857. 

D'ARREST     .       -       - 

1872 

Pretty  uniformly  filled  with  faint  nebulosity. 

WINLOCK     .     .      ; 
TROUVELOT  .     . 

1874 

Pretty  uniformly  black. 

TEMPEL  .     .     - 
HOLDEX.     .     .     - 

1876 

1877 

Do. 
All  north  of  M  =  47"  filled  with  faint  nebulosity,  except,  of  course,  the  dark  streak  on  its 
preceding  edge.     South  of  this  pretty  black,  but  less  so  than  r'. 

No  change  in  r  except  with  regard  to  ScHROETEB'ssecond  bridge,  q.  v 


2l6 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

The  second  bridge  o/ SCHROETER  (in  r). 


Observer. 

Date. 

Remarks. 

SCHROETER  .       .       . 

1797 

Jan.  25  (see  Fig.  13).     The  second  bridge  is  plainly  laid  down. 

HERSCHEL   .     .     . 

1826 

The  north  half  is  plainly  indicated. 

HERSCHEL   .     .     . 

1837 

No  bridge  laid  down. 

LAMONT  .... 

1837 

Do. 

-DE  Vico.     .     .     . 

183?" 

Do. 

LASSELL.     .     .     . 

1847 

Do. 

W.  C.  BOND 

1848 

Do. 

LIAPONOFF  . 

ft«M|fl? 

1851 

Do. 

LASSELL  . 

1854 

Do. 

SCHMIDT      .     .     . 

1861 

Do. 

LASSELL  .... 

1862 

The  north  one-third  is  plainly  indicated. 

WEBB     .... 

1863 

Not  laid  clown. 

G.  P.  BOND  .     .     . 

1865 

No  bridge  laid  down  in  engraving,  but  plainly  drawn  on  several  different  charts.    . 

WEBB           .,    . 

1866 

No  bridge  laid  down. 

ROSSE     .... 

1867 

A  second  bridge  without  nucleus  is  laid  down. 

SECCHI    .... 

1868 

No  bridge  laid  down. 

D'ARREST    . 

1872 

Do. 

WINLOCK     .     .      } 

TROUVELOT  .     .      5 

1874 

Do. 

TEMPEL  .... 

1876 

Do. 

HOLDEN.     .     .     . 

1877 

Second  bridge,  as  in  LASSELL,  1862,  and  SCHROETER,  1794. 

SCHROETER,  1797,  lays  down  the  second  bridge  in  r,  HERSCHEL  in  England  (1826) 
shows  it.  Having  this  first  drawing  before  him  in  1837  at  the  Cape  of  Good  Hope, 
he  does  not  show  it.  LASSELL,  1862  (4-foot  reflector),  shows  it  faint.  BOND  (1865) 
does  not  show  it  in  the  engraving,  but  it  is  faintly  but  plainly  shown  in  his  drawings. 
ROSSE  (1867)  shows  it.  In  1874-1875  I  did  not  see  it,  although  r  was  examined  in 
1875  carefully.  It  was  first  seen  here  in  1876.  It  is  now  brighter  than  at  that  time. 
These  facts,  taken  in  connection  with  the  general  details  given  under  the  heading  r, 
seem  to  me  to  prove  a  change  in  its  brightness ;  for  how  otherwise  could  SCHROETER 
see  in  1797  what  escaped  HERSCHEL  (1837)  and  LASSELL  (1854)  with  superior 
advantages  I 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

Pons  tSchroeteri  and 


217 


Pom  S. 

8+ 

(  >bserver. 

Date. 



Remarks. 

P 

A  a. 

61 

Noi  th  half  alone  given. 

o 
1826          175 

1837          163.5 

72 

HEUSCHEI.   .     .     . 

LAMONT       .     .     . 

1837 

80 

52 

[The  south  end  of  fans  .SV/mvAv/is  28"  noith  of  6']  [gv 

not  given]. 

LAMONT 

1837      [160??] 

'-     • 

LASSELI.       .     . 

1847        180?? 

-     - 

-      - 

W.  C.  BOND      .     . 

1848         154 

-      - 

LlAI'ONOKF*. 

1851 

71.9 

42.3 

In  the  direction  of  669  and  685  (text,  p.  79). 

I.  \SSEI.I.          .       .        . 

1854     ,    180?? 

• 

• 

SCHMIDT      .     .     . 

1861          139 

-      - 

No  nucleus;  the  north  two-thirds  only  are  laid  down. 

STRUVK  .... 

1  86  1 

The  fons  crosses  the  whole  of  Sinus  magnus,  and  & 
_      the  brightest  part  ;  but  ^,  is  sometimes  wanting. 

is  usually 

LASS  E  r.  i,       .     .     . 

1862         175 

V 

No  nucleus  ;  the  north  two-thirds  only  are  laid  down. 

WEIili        .... 

1863         .     . 

(  )n  the  line  joining  669  and  685.     gu  not  drawn. 

G.  P.  BOND  .     .     . 

1865          157-5        7°             44           Bridge  continuous  ;  break  nearly  as  in  ROSSF.  in  various  drawings. 

WEIIB      .... 

1866          .     . 



Parallel  to  line  669-685,  but  following  this  line  a  little. 

R<  ISSE        .... 

1867          175 
1868          .      . 

7o            39 

Length  of  bridge  78".     There  is  a  decided  break  in  the  bridge 
about  its  middle  point. 
tfi  formerly  visible,  but  not  seen  in  1868  (text,  p.  19).     In   1857 
the  /WAT  Schrot-tt'ri  rvwrr.r  towards  the  west.     In  1867  conca-'f. 

SECCHI    .... 

D'ARREST     .       .        . 

1872          175 

Length  of  bridge  40''. 

WlNLOCK     .       -       -   ) 

TROUVKI.OT     .     .  > 

1874          168.2 

Perhaps  a  break  indicated.     ,<>  not  given. 

TKMI-EI  

jg-5                                          .     .        Nucleus  very  much  brighter  than  the  rest. 

HOI.DEN       .     .     . 

1877          170.2          77.4          40.8      There  is  a  break  on  each  side  of  .-•b,  one  in  A5=  22",  and  the 
other  in  A  5  =  60".    .v,,  stellar  at  first  ;  not  so  later  in  the  work. 

Variations  in  brightness  seem  to  me  to  be  proved,  as  remarked  by  Om> 
and  SECCHI.     See  their  memoirs  as  previously  referred  to. 

A  Tvr,      \T                »>i 

STRIA  r 

218 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

G  and  lacm  Lassellii. 


Observer. 


South      s.  f.  bright 
edge.          point. 


SCHROKTER 
IlERSCHEL. 

HKRSCHEI.. 
LAMONT  . 

DE  Vico     . 
LASSELI. 
W.  C. BOND 

LlAl'ONOFF 

LASSKLL  . 
SCHMIDT  . 

STRUVE 
LASSEU, 

G.   P.  B«>M> 

WEBH  .  . 
ROSSE  .  . 
SKCCHI . 


D'ARREST  . 

WIN  LOCK  . 
TROUVEI.OT 

TEMPEI. 
HOI.DEN 


] )ate. 


1794 
1826 

1837 
1837 

1839 
1847 
1848 
1851 

1854 

1861 


1861 

1862   90.4 

1865  95.0 

1866  •..; 

1867  91.0 

1868 


Lacus  Lassellii. 


p        Art      A<5 


68 1  on  following  edge 

152        117.5     68 1  in  middle  of  6.     South  edge  of  6 
i  suddenly  much  brighter. 

.   :  681  on  following  edge 

[681  on  follmping  t&gt\      .... 
-   ;  No  bright  sharp  following  point. 


Discovered  lactis  Lassellii. 

Plainly  laid  down. 

Plainly  laid  down  though  different  from 

its  present  appearance. 
No  Incus  Lassellii. 
No  lams  Lassellii. 


143-6       71-5 


1872       90 
|     1874 


1876  !   -     - 

1877  I    89.7 


Concave  towards  the  south ;  south  edge 
brighter. 

No  sharp  following  point,  at  least  not 
as  in  BOND. 


136.8      68.4  ; 

I5°         83        South  edge  convex  towards  soiith  if  at 

all;  nearly  straight. 
-      •       -      -   i  See  Notes. 
144       81-87 
192.8       79.6     Roughly   speaking  the  preceding  half 

^>  folloiving  half;    a  //////•  brighter 

on  the  south  edge,  but  not  much ; 

its  south  shore  is  "directed  towards 

the  north." 
192          55        South  edge  concave  towards  the  south. 

170  76.3  General  shape  as  in  Naval  Observa- 
tory drawing,  but  brightness  dif- 
ferent. 

.    i  Following  half  ]>  preceding  half  . 
79     :  The  preceding  half  ^>  folltmnng  half. 


Does  not  extend  quite  to  Siiinsi/iagnns. 

Connects  with  Sinus  m<ignns. 

Does  not  connect  with  Sinus  HIU^/IIIS. 


Laci<s  Lassellii  is  continuous  to  Sinus 
mngnus  on  moonlightnights,  but  in- 
terrupted on  dark  nights. 


165 


Connects  with  Sinus  magnits. 

Ditto,  only  on  moonlight  nights,  but 
the  south  part  of  it  is  in  fact  filled 
with  faint  nebulosity  which  moon- 
light destroys. 


Possibly  a  change  in  Jd  since  HERSCHEL  (1837). 

Angle  between  the  line  685-741  and  the  north  shore  of  Sinus  maavus. 
I  have  thought  it  worth  while  to  tabulate  the  values  of  this  angle,  as  follows : 

O  O          f 

The  angle  is  from  5°  to  10°  for — 

WINLOCK  and  TROUVELOT  (1874). 

W.  C.  BOND  (1848). 

LIAPONOFF  and  ROSSE  (1857  and  1867). 

D'ARREST  (1872). 

HERSCHEL  (1824). 

HUYGHENS  (1656). 

HOLDEN  (1877). 

It  is  15°  for  PICAR-D  (1673). 

It  is  30°  for  HERSCITEL  (1837);  LAMONT'S  (1837)  drawing  confirms  HERSCHEL. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


219 


It  will  be  noted  that  HERSCHEL  (1837)  and  LAMONT  (1837)  are  strangely  different 
from  all  the  rest,  and  that  they  agree.  The  discrepancy  is  too  great  to  explain  by 
small  errors.  If  this  is  an  error  it  is  a  gross  one. 


Observer. 


HERSCHEL  . 
HERSCHEL  . 
LAMONT. 
W.  C.  BOND 

LASSELL . 
SCHMIDT 
LASSELL.  . 

WEBB 

G.  P.  BOND  . 

WKBB  .  . 
ROSSE  .  . 
SECCHI  .  . 

D'ARRKST     . 

WIN  LOCK  . 
TROUVELOT  . 

T KM TEL  . 
I  I  OLDEN. 


Date. 

1826 
1837 
1837 
1848 

1854 
1861 
1862 
1863 
1865 

1866 
1867 
1868 
1872 


Remarks. 


All  equally  black. 

Do. 

Not  laid  down. 

South  part  brighter  than  north  part. 
Not  entirely  black. 

faintly  nebulous.     It  is  prolhible  that  SCHMIDT  lias  seen  a  and  part  of  <>  n  near  TT  as  connected. 
Black. 

Do. 

Completely  black.     See  Man.  Not.,  R.  \.  S.,  vol.  24,  p.  178.     Its  south  border  is   10"  or 

12"  south  of  ff. 
Black. 

The  north  half  the  brighter. 
Not  laid  down. 
Filled  with  faint  nebulosity. 


1874         Following  half  brighter  than  preceding  half. 
\ 

1876          South  part  nebulous;   north  part  black. 
1876         North  part  brighter,  but  all  very  dark. 


Probably  constant. 


Q,  P,  R. 


Observer. 

Date. 

North 
edges. 

Remarks. 

/ 

• 

o 

HERSCHEL  .     .     . 

1837 

93-95 

LAMONT.     .     .     . 

1837 

1045? 

[R,  P,  and  S  not  laid  down.] 

DEVico.     .     .     . 

1839 

117? 

\V.  C.  BOND     .     . 

1848 

97-5 

Cuts  off  a  little  of  R. 

SCHMIDT 

1861 

105  ? 

LASSELI  

1862 

98          Cuts  off  a  little  of  R. 

G.  P.  BOND  .     .     . 

1865 

115                    Do. 

ROSSE     .... 

1867 
1868 

101.0?    Cuts  off  1\. 
1  Angle  not  easily  measurable.     It  does  nut  agree  with  Kuv-h  and  r.<>\n. 

SKCCIII    .... 

D'ARREST     .       .       . 

1872 

i»5 

WIN  LOCK      .       -        ) 

TROUVELOT  .     .      > 

1874 

97.0 

Cuts  off  a  little  of  K. 

HOLDEN.       -       -       - 

1876 

105-7 

j 

Probably  constant. 


220 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


Q. 


Observer. 

Date. 

Following  point. 

Remarks. 

Aa 

A8 

n 

n 

LE  GENTIL  .     .     . 

1758 

. 

. 

Same  R.  A.  as  708. 

LAMONT  .... 

1837 

162.4 

40.  2? 

[671  on  north  edge  of  brightest  nebulosity,  or  a  little  north  of  the  edge  in 
a  slightly  fainter  part.] 

LASSELL  .... 

1847.' 

-     - 

-       - 

W.  C.  BOND.     .     . 

1848 

"150 

[Same  R.  A.  as  708.]     *A  diagram  of  1848.  Jan.  17,  gives  150".     Annals 
If  air.  Coll.,  v,  p.  176. 

LlAPONOFF    .       .       - 

1851 

146.5 

22.  O 

Precedes  708  by  4".6.     Cj  well  defined. 

SCHMIDT      .     -     . 

1861 

-     - 

-       - 

Follows  708  by  from  7"  to  8". 

LASSELL.     .     .     . 

1862 

124.3 

6.8 

Same  R.  A.  as  7°8. 

WEBB      .... 

1863 

-     - 

-     - 

The  line  joining  669  and  the  extreme  point  of  Q  cuts  the  line  708-741  at 

G.  P.  BOND  .     .     . 

1865 

145 

20 

??ths  of  its  length  measured  from  708.     Q,  N,  O,  P  quite  bright. 
46 

Q  and.N  quite  bright. 

WEBB     .... 

1866 

-     - 

-     - 

Appears  to  have  its  following  point  in  same  R.  A.  as  708.     Q,  X,  O,  P 
are  all  fainter  than  E,  F,  G,  I,  H,  D,  and  [A  ?] 

ROSSE     .... 

1867 

US 

21 

SECCHI    .... 

1868 

153-5 

.       - 

D'ARREST     .       .       . 

1872 

192 

32 

WlNLOCK      .       -        •) 

TROUVELOT       -      > 

1874 

155 

-       - 

TEMPEI  

1876 

. 

. 

. 

Same  R.  A.  as  708. 

HOLDEN.     .     .     . 

1877 

150 

28 

Do. 

Probably  constant. 

T"  and 


Observer.                Date. 

r". 

«• 

HERSCHEL  .     .     .       1826 
HERSCHEL  .     .     .       1837 

Perfectly  black    
Filled  with  faint  nebulosity  

Perfectly  black. 
Almost  empty  of  nebulosity. 

LAMONT.     .     .     .       1837 

Same.                    . 

DE  Vico      .     .     .  !     1839 

Black. 

LASSELL.     .     .     .       1847 
W.  C.  BOND.     .     .       1848 

Filled  with  faint  nebulosity 

Full  of  curdled  nebulosity. 

LASSELI  1854 
SCHMIDT      .     .     .       1861 
LASSELL.     .     .     .       1862 
G.  P.  BOND  .     .     .       1865 

ROSSE     ....       1867 

Contains  nebulosity  
Not  laid  down      
Almost  totally  black  
Completely  black.     See  Mon.  Not.,  R.  A.  S., 
vol.  24,  p.  1  78. 
.  do 

Contains  nebulosity. 
Pretty  nearly  totally  black. 
Nebulous  particularly  u\\Jollcwiti£  one  half. 
Faintly  nebulous. 

Very  faintly  nebulous. 

D'ARREST    .     .     .       1872 

WlNLOCK                       > 

1874 

Uniformly  filled  with  faint  nebulosity. 
Same   

Very  faint  nebulosity. 

TROUVELOT.     .      > 
TEMPEL  ....       1876 

Black   

Nebulous. 

HOLDEN.     .     .     .       1877 

Quite  dark  —  empty  

Filled  with  nebulosity,  except  a  black  streak 
close  to  o  TT. 

The  various  evidence  points  to  changes  in  brightness. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

07T. 


221 


Observer. 

Date. 

Whole 
mass. 

Remarks. 

P 

HERSCHEL  .     .     . 

1826 

o 

\ 

No  nucleus  o,  but  a  detached  brighter  spot  about  where  n  now  i.s. 

HERSCHEI,  .     .     . 

i839 

no? 

No  nuclei  o  and  it. 

W.  C.  BOND.     .     . 

1848 

99-5 

No  nuclei  o  and  it. 

LASSKLL  .... 

1862 

90 

No  nuclei  o  and  n  ;  at  the  follmvin*  end  it  joins  <|iiite  marked  nebulosity. 

G.  P.  BOND.     .     . 

1865 

93  ? 

In  original  drawing  from  which  engraving  was  made/  —  90  ±. 

ROSSE     .     .     .     . 
WINI.OCK    .     .     .  j 
TROUVEI.OT     .     .  > 

1867 

1874 

93-o 
90  ± 

No  nucleus  o;  the  nucleus  it  in  A«=  144". 
o  and  it  not  laid  down. 

TEMPEI  

1876 

The  mass  on  is  laid  down. 

IIol.UKN          .        .       . 

1875 

1 

it  ^>  o;  both  stellar  in  appearance,  but  not  so  much  so  as  ;•„. 
. 

Probably  constant. 

SPECTROSCOPIC  OBSERVATIONS- OF  THE  NEBULA. 

The  spectroscopic  apparatus  of  the  Naval  Observatory  and  the  relation  of  aper- 
ture and  focal  length  of  the  26-inch  equatorial  were  such  that  no  advantage  could 
have  been  gained  by  devoting  time  to  the  re-examination  of  the  spectrum  of  the 
nebula.  A  few  extracts  from  work  done  by  other  astronomers,  notably  HUGGINS, 
SECCHI,  and  VOGEL  are  given  herewith,  and  for  similar  observations  reference  is  made 
to  the  original  papers,  whose  titles  will  be  found  in  the  List  of  Books  and  Memoirs  at 
the  end  of  the  Introduction.  The  most  important  of  these  is  the  paper  of  Profess,. r 
D'ARREST  published  in  1872.  The  main  facts  to  be  noted  seem  to  be  that  this  nebula 
is  certainly  gaseous  and  similar  in  constitution  to  other  gaseous  nebulae.  The  changes 
made  out  in  the  brightness  of  its  parts  are  thus  more  credible  than  if  it  were  a  true 
stellar  nebula,  for,  as  is  remarked  by  GEORGE  BOND,  "the  variability  of  nebuliv,  it' 
dependent  on  the  variability  of  the  stars  of  which  they  may  be  supposed  to  be  formed, 
would  require  the  greater  portion  of  these  stars  to  grow  faint  at  one  and  the  same- 
time.  The  improbability  of  this  explanation  is  greatest  where  the  number  of  stars  of 
which  the  nebula  is  composed  is  the  greatest,  i.  e,  where  it  is  brightest"  The  gaseous 
nature  of  some,  at  least  of  the  small  stars  near  the  trapezium,  is,  I  think,  indicated 
by  their  peculiar  behavior  under  different  magnifying  powers.  As  has  been  before 
remarked  most  of  them  are  best  seen  with  low  powers. 

The  space  within  about  the  trapezium  is  really  nebulous  and  not  void, 
was  first  suspected  by  BOND  and  HERSCHKI,     The  spectrum  of  the  nebula  seem*  to 
consist  of  four  lines.     The  positions  of  the  three  brightest  are  about 

A  5004 
B  495-8 
C 


222  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

These  seem  to  indicate  the  presence  of  hydrogen  and  nitrogen  in  the  nebula, 
and  that  these  are  its  principal  constituents.  We  have  yet  to  learn  the  true  inter- 
pretation of  the  third  line  in  the  spectrum.  A  fourth  line  Hy  has  been  occasion- 
ally seen. 

Dr.  HUGGINS'  paper  in  the  Proceedings  of  the  Royal  Society,  1865,  January  26, 
contains  the  important  remark  that  "the  positions  in  the  spectra  of  <*,  /?,  yj  £,  trapezii, 
which  correspond  to  the  positions  in  the  spectrum  of  the  three  lines  of  the  nebula 
were  carefully  examined,  but  in  no  one  of  them  were  dark  lines  of  absorption 
detected"  as  might  have  been  expected  if  the  nebula  was  nearer  to  us  than  these 
four  stars. 

In  Dr.  HUGGINS'  paper  u  on  the  spectra  of  some  of  the  stars  arid  nebula,"*  he 
describes  at  some  length  his  later  observations  of  the  nebula  of  Orion  (p.  541),  and 
corroborates  his  former  measures,  and  after  some  general  considerations  as  to  the 
extinction  of  light  in  space,  says  (p.  544),  "  the  result  of  the  re-examination  of  the 
spectrum  of  this  nebula  appears  to  give  increased  probability  to  the  suggestion  * 
*  *  that  the  substances  hydrogen  and  nitrogen  are  the  principal  constituents."  *  *  * 
"  I  am  still  unable  to  find  any  terrestrial  line  which  corresponds  to  the  middle  line." 

Dr.  HuGGiNst  shows  that  the  nebula  of  Orion  is  not  receding  from  the  earth  with 
a  velocity  greater  than  10  miles  per  second,  nor  approaching  the  earth  faster  than  20 
or  25  miles  per  second.  In  general,  nebulae  have  not  shown  motions  to  or  from  the 
earth  as  the  fixed  stars  have.  In  the  Proceedings  of  the  Royal  Society  for  1874,  March 
26,  Dr.  HUGGINS  returns  to  this  question.  His  results  are  the  same  except  that  it  is 
possible  that  the  Orion  nebula  may  be  approaching  the  earth  as  fast  as  30  miles  per 
second  approximately.  v 

Mr.  LE  SUBURB  in  reporting  his  measures  of  the  spectrum  of  the  Orion  nebula, 
states  that  the  nebulosity  within  the  trapezium  is  comparable  in  brightness  with  that 
immediately  surrounding  it,  as  is  shown  by  the  relative  brightness  of  the  spectra. 

In  the  Proceedings  of  the  Royal  Society  (1872,  p.  383),  Dr.  HUGGINS  gives  the 
results  of  a  new  examination  of  the  spectrum.  The  principal  point  to  be  noted  is 
that  it  is  possible  that  the  brightness  of  the  3d  (and  4th)  lines  may  vary  in  brightness 
relative  to  lines  i  and  2  from  time  to  time. 

SECCHI,§  after  describing  his  previous  observations  of  the  spectrum  of  this  nebula 
in  which  the  line  H/?  of  hydrogen  was  seen  while  none  of  the  other  characteristic 
lines  of  the  substance  could  be  observed,  examines  the  question  as  to  whether  the 
presence  of  this  line  indicates  that  hydrogen  is  indeed  present  in  the  nebula. 

A  Geissler  tube  containing  hydrogen  gave,  in  his  spectroscope,  the  three  lines  Ho:, 
H/?,  H.y ;  when  the  light  from  this  tube  was  enfeebled  by  reflection  before  entering 
the  spectroscope  only  one  line,  H/?,  was  seen.  He  concludes,  first,  that  the  presence 
of  a  single  line  is  sufficient  to  prove  the  existence  of  an  elementary  substance  in  a 
celestial  body ;  second,  that  the  monochromatic  nature  of  the  light  of  nebulae  is 
probably  only  apparent,  and  that  there  are  probably  other  spectral  lines  not  seen 

Philosophical  Trun suctions,  1868,  p.  529. 
t  Proceedings  of  the  Royal  Society,  1868,  May  14,  p.  384. 
t  Proceedings  of  the  Royal  Society,  1870,  March  3,  p.  242. 
§  In  the  Comptes  Rendus,  vol.  66,  p.  643. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  223 

on  account  of  their  faintness ;  third,  that,  as  we  do  not  see  certain  lines  whose  abso- 
lute brilliancy  is  greater  than  H/?,  we  may  conclude  that  certain  substances  are 
not  present  in  the  nebula;  and  fourth,  that  substances  there  present  act  by  radia- 
tion  and  not  by  absorption,  as  in  stars. 

Dr.  VOGEL*  gives  the  results  of  four  nights'  observations  in  1871  on  the  spectrum, 
as  below : 

1.  W.  L  =  500.4  M.  M. 

2.  =495-8        " 
3-  =486.1        " 

The  uncertainty  is  about  db  o.i  5  M.  M.  The  first  line  is  the  brightest,  the  second 
the  faintest;  no  fourth  line  was  seen.  The  different  parts  of  the  nebula  gave  ahvuv- 
the  same  spectrum,  and  the  relative  brightness  of  the  three  lines  was  always  the  same. 
The  first  line  coincides  with  a  double  line  in  the  atmospheric  spectrum.  The  second 
lino  does  not  agree  in  position  with  the  lines  of  any  known  terrestrial  substance. 
The  third  line  coincides  with  the  hydrogen  line  H/?. 

Dr.  BREDicuiNf  gives  as  the  normal  spectrum  of  gaseous  nebula1,  the  following 
positions  of  the  three  brightest  lines : 

A  =  5003.9  ±  1.2 
B  =  4957.9  db  114 
C  =4859.2  dz  3-1 

The  probable  errors  are  computed  by  supposing  the  nebula3  G.  C.  4964,  4628, 
4234,  4447,  4390,  4510,  and  4373  to  have  the  same  spectrum,  in  fact,  and  the  small 
differences  to  be  due  to  accidental  errors  of  observation. 


*  Ast.  Nacli.,  p.  78,  col.  245. 

t  In  the  Moscow  Observations,  vol.  ii,  p.  60. 


224  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

* 

PART  IV. — CONCLUSIONS  TO  BE  DERIVED  FROM  THE  FOREGOING  MEMOIR. 

It  may  be  well  to  summarize  the  foregoing  work,  in  order  to  review,  briefly,  the 
ground  over  which  we  have  gone  and  the  conclusions  which  are  to  be  gained.  The 
object  of  the  work  was  twofold :  First,  to  make  such  a  detailed  study  and  description 
of  the  central  and  brighter  portions  of  this  nebula,  that  a  repetition  of  the  work  would 
be  easy  and  short,  and  so  that  the  question  of  any  future  change  in  the  parts  con- 
sidered can  be  settled  definitely  and  beyond  a  doubt  and  without  any  great  labor. 
The  form  in  which  the  observations  are  classified  in  Part  II  seems  to  me  to  satisfy  this 
condition.  The  accuracy  .of  the  micrometer  measures  is  sufficient  for  the  purpose, 
and  greater  than  I  anticipated ;  for  example,  the  JS  of  the  brightest  part  of  F  is 
—  io2//.o±o//.3  (5  nights);  of  the  brightest  part  of  G  is  —  65". 3  ±  o".8  (6  nights); 
the  position-angle  of  the  frons  is  50°. 3  ±  o°.5  (4  nights),  and  so  on.  These  are  sur- 
faces and  not  points  it  will  be  remembered.  It  will  be  noted  that  the  first  object  of 
my  work  is  precisely  that  proposed  to  himself  by  LE  GENTIL  in  1758.  The  second 
object  was  to  completely  and  thoroughly  discuss  the  large  mass  of  material  already 
on  hand  derived  from  the  observations  of  224  years  (1656-1880).  All  available 
drawings  were  examined,  and  thirty-eight  are  here  engraved  (nearly  all  on  the  same 
scale),  and  abstracts  have  been  made  of  all  available  observations  arid  are  here  given. 
Several  unpublished  series  and  drawings  have  been  printed  for  the  first  time,  notably 
those  of  LASSELL,  SCHMIDT,  and  LANGLEY.  By  this  examination  the  epoch  of  the  first 
trustworthy  observations  has  been  carried  back  from  1824  to  1758.  LE  GENTIL'S 
figure  of  the  central  part  yields  evidence  comparable  in  value  with  the  first  figure  of 
Sir  JOHN  HERSCHEL. 

I  conclude  that  there  have  been — 

(1)  possible  changes  of  brightness  in  the  masses  of  J  and  15; 

(2)  changes  in  the  brightness  of  A  ; 

(3)  changes  in  the  brightness  of  E  ; 

(4)  changes  in  the  brightness  of  D ; 

(5)  undoubted  change  in  the  brightness  of  SCHROETER'S  second  bridge ; 

(6)  undoubted  changes  of  brightness  in  SCHROETER'S  bridge  and  in  the  appear- 
ance of  r/0,  its  nucleus  ; 

(7)  a  possible  change  in  the  position  of  the  south  edge  of  G  since  1837 ; 

(8)  a  probable  change  in  the  brightness  of  T"  • 

(9)  a  probable  change  in  the  brightness  of  £ ;. 

(10)  a  certain  change  in  the  development  of  the  mass  //  near  D. 

There  is  no  evidence  whatever  for  any  change  of  form  other  than  that  which  may 
be  due  to  such  changes  of  brightness ;  as  in  the  cases  of  A,  D,  //,  etc  I  do  not  find 
any  change  of  the  Messierian  branch  near  793. 

The  connection  of  the  stars  of  the  trapezium  with  the  nebula  appears  to  me  to  bo 
settled  by  the  conclusions  of  my  paper  reducing  Professor  HALL'S  observations  of 
these  stars,  and  by  various  former  observations,  such  as  the  important  one  by  Dr 
HUGGINS-  of  those  portions  of  the  spectra  of  a,  /?,  ;/,  $,  trapezii,  near  the  places  of  the 
nebula  spectral  lines  i,  2,  3,  and  others. 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  225 

The  change  in  the  brightness  of  D,  h,  and  other  masses,  is  shown  by  the  Wash- 
ington Observations  taken  alone.  These  also  show  the  feasibility  of  making  tolerably 
accurate  photometric  observations  of  the  relative  brightness  of  two  nebulous  masses. 
Certain  of  the  masses  have  varied  in  brightness  during  the  period  of  observations.  A 
new  nebulous  patch  (h)  has  been  seen  from  the  time  of  its  origin,  when  it  was  stellar 
in  appearance  and  faint,  until  now,  when  it  is  bright  and  of  measurable  dimensions. 

It  appears  to  me,  then,  to  have  been  shown  that  the  figure  of  the  nebula  of  Orion 
has  remained  the  same  from  1758  to  now  (if  we  except  a  change  in  the  shape  of  its 
apex  (E)  about  1770,  which  appears  quite  possible)  ;  but  that  in  the  brightness  of.  its 
parts  undoubted  variations  have  taken  place,  and  that  such  changes  are  even  now 
going  on. 

I  have  not  hesitated  to  give  the  conclusions  to  which  I  have  been  led  in  the  course 
of  this  work,  although  I  am  aware  that  all  of  these  may  not  be  accepted  on  a  first 
reading. 

With  regard  to  any  subject  of  this  kind,  every  competent  judge  has  a  body  of 
opinion  derived '  partly  from  his  own  experience  and  partly  from  judgments  formed 
from  time  to  time  by  examinations  of  the  work  of  others.  In  general,  this  body  of 
opinion  leans  to  the  view  that  the  phenomena  presented  by  the  celestial  bodies,  are, 
for  long  periods  of  time,  quite  constant.  For  example,  accounts  of  supposed  changes 
in  the  conformation  of  the  lunar  craters  are  received  and  rightly  received  with  a 
measure  of  grave  doubt,  and  yet  no  one  is  disposed  to  deny  that  real  changes  are  now 
taking  place  from  moment  to  moment,  just  as  they  have  in  the  past;  but  each  par- 
ticular recorded  evidence  of  change  is  regarded  with  doubt,  and  a  full  measure  of 
proof,  depending  on  sufficient  observation,  is  justly  demanded.  A  competent  observer 
is,  however,  still  bound  to  put  his  observations  on  record. 

It  appears  to  me  that  I  have  less  reason  to  hesitate  in  recording  my  own  judg- 
ments upon  the  phenomena  here  described,  as  the  observations  themselves  are  given 
in  full  detail,  and  the  materials  for  an  adequate  judgment  are  spread  out  for  inspection. 

At  least,  I  can  be  sure  that  all  the  existing  evidence  is  impartially  presented  in 
such  a  way  as  to  be  readily  added  to  in  the  future,  and  I  cannot  myself  doubt  but  that 
the  principal  conclusions  here  set  down  will  be  confirmed  by  others. 
APP.  V -29 


226  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

ADDENDUM. 
PHOTOGRAPHIC  RESULTS  OF  DR.  HENRY  DRAPER. 

The  first  photograph  of  the  nebula  of  Orion  was  made  by  Dr.  HENRY  DRAPER  in 
September,  I88o,  and  the  unavoidable  delay  which  has  occurred  in  printing-  the  present 
memoir  enables  me  to  include  an  account  of  the  astonishing  results  which  he  has 
attained.  A  wood-cut  which  I  had  prepared  from  his  first  photograph  was  found  to  be 
so  unsatisfactory  thkt  Dr.  DRAPER  most  generously  offered  to  supply  the  necessary 
photolithographic  reproductions  of  his  last  negative  (taken  March  14,  1882)  to  accom- 
pany the  brief  account  I  had  prepared.  The  full  page  photolithograph  is  here  given 
as  figure  40. 

I  requested  Dr.  DRAPER  to  prepare  some  account  of  his  work  to  be  presented  with 
it,  and  I  print  below  a  memorandum  which  he  has  kindly  furnished. 

"MEMORANDUM  TO  ACCOMPANY  THE  PHOTOGRAPH  OF  THE  NEBULA  IN  ORION 
SENT  TO  PROFESSOR  HOLDEN  FOR  HIS  MEMOIR. 

BY  HENRY  DRAPER,  M.  D.  • 

"As  far  as  I  know,  no  photograph  of  any  nebula  has  been  taken  except  in  my 
observatory.  The  first  photograph  of  the  nebula  in  Orion  was  made  on  September  30, 
1880,  with  rny  CLARK  telescope  of  1 1  inches  aperture  and  an  exposure  of  51  minutes. 
It  comprised  the  brightest  parts  of  the  region  in  the  neighborhood  of  the  trapezium 
and  showed  the  condensed  masses  well.  In  March,  1881,  a  number  of  photographs 
of  this  object  were  taken,  the  best  being  on  March  1 1  with  an  exposure  of  104  minutes. 
By  comparison  with  the  former  picture  this  made  a  marked  advance,  and  minute  stars 
down  to  the  14.7  magnitude  of  POGSON'S  scale  were  shown.  An  account  of  it  was 
read  before  the  French  Academy  of  Sciences  and  printed  in  the  Comptes  Hendus,  April 
1 8,  1881. 

"On  March  14,  1882,  the  negative  was  made  from  which  the  photolithographic 
enlargement  in  this  memoir  was  produced.  The  instrument  used  was  the  CLARK  tele- 
scope of  1 1  inches  aperture  mounted  on  the  equatorial  stand  and  driven  by  the  clock 
which  I  had  constructed.  The  exposure  was  from  7h  o8m  to  9h  25™;  that  is,  137 
minutes:  gelatino-bromide  plates  were  employed.  The  night  was  clear  but  cold  and 
windy.  The  mean  temperature  was  27°  Fahr.;  the  wind  NNW.  and  in  gusts,  the 
strongest  pressure  being  5  pounds  per  square  foot  about  nine  o'clock;  the  whole  travel 
of  the  wind  during  the  exposure  was  35  miles.  The  variation  in  the  force  of  the  wind 
is  one  reason  why  the  stars  show  some  ellipticity  under  this  magnifying  power;  the 
gusts  of  course  displaced  the  telescope  somewhat,  though  the  mounting  is  firm  and  the 
clock-work  strong. 

"In  the  photograph  the  larger  stars  are  much  overexposed,  the  proper  time  to 
make  a  good  picture  of  the  trapezium  being  about  2  minutes.  The  twinkling  of  these 
stars  is  therefore  recorded  on  the  sensitive  plate,  and  gives  to  them  an  excess  of  size. 
If  a  photograph  should  be  taken  on  a  steady  night  the  stars  of  the  trapezium  would 
be  easily  separated,  and  in  the  original  negative  of  this  picture,  in  a  strong  light,  the 
separation  can  be  seen.  The  variation  in  size  of  the  stellar  images  gives  an  idea  of 


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FHUTUGRAFH  DF  THE  NEBULA  IN  DRICN. 
Taken   by   Professor   Henry  Draper   M.D,  .March   14th  1BB2,   Exposure 

3n  artotypB  enlargement  by  E,  Bierstadt  fram  **  Dri^  »f  ^  ™*la 
brighter  then  the  nebula  are   overexposed,  This  picture  when  compared  with  that  taken 
increased  extent   and  shows  stars  of  the  14,7  magnitude   of  Fo??nn's,  scale. 

Fie, 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OP  ORION.  227 

the  relative  magnitude  of  the  stars,  though  that  estimate  requires  correction  for  the 
color  of  the  stars.  It  must  be  remembered  that  no  one  enlargement  can  do  justice  to 
the  original  negative;  various  exposures,  various  intensities  of  light,  and  various  points 
of  view  are  necessary  for  a  complete  examination. 

"During  the  month  of  March,  1882, 1  also  made  four  photographs  of  the  spectrum 
of  the  nebula  in  Orion,  which  are  described  in  the  number  of  the  American  Journal  of 
Science  for  May,  1882.  Two  of  these  were  made  with  the  slit  spectroscope  that  I 
usually  employ  for  photographing  spectra  of  the  stars  and  they  show  two  lines  in  the 
ultra-violet  plainly,  beside  the  traces  of  two  others.  The  first-mentioned  two  are 
hydrogen  7,  A  4340,  and  hydrogen  6,  A  4101 ;  the  others  are  too  faint  to  give  a  good 
estimate  of  the  wave  length. 

"The  other  spectrum  photographs,  taken  without  a  slit,  show  that  two  of  the  con- 
densed masses  preceding  the  trapezium  give  a  continuous  spectrum,  and,  therefore, 
contain  either  gas  under  pressure,  or  liquid,  or  solid  matter.* 
"271  MADISON  AVENUE,  New  York,  April  29,  1882." 

Although  it  is  still  too  soon  to  give  a  final  discussion  to  the  photographic  results 
attained  by  Dr.  DRAPER,  I  cannot  refrain  from  pointing  out  some  of  the  conclusions 
which  may  be  drawn  from  this  marvelously  perfect  representation  of  the  nebula. 

If  we  compare  the  frontispiece  with  Fig.  40  we  shall  be  able  best  to  appreciate 
the  important  advance  which  has  been  made.  BOND'S  engraving  is  the  most  accurate 
drawing  that  has  been  made,  even  as  a  map,  and  as  a  picture  it  is  decidedly  the  best 
representation  of  a  single  celestial  object  which  we  have  by  the  old  methods.  The 
work  of  observing  alone  extended  over  years  and  consumed  many  precious  hours.  I 
have  before  said  how  much  labor  was  spent  upon  the  mechanical  execution  of  the  steel 
plate;  scores  of  revises  were  criticised  and  read. 

Dr.  DRAPER'S  negative  was  made  in  137  minutes,  and  for  nearly  every  purpose 
is  incomparably  better  than  the  other.  The  color  and  tint  of  the  nebula,  which  is 
wonderfully  preserved  in  BOND'S  engraving,  is  lost  in  the  photograph;  and  yet,  if  the 
latter  is  held  up  between  the  eye  and  a  window,  the  pictorial  effect  is  most  striking. 

The  amount  of  preparation  for  the  two  works  is  not  to  be  estimated  by  years  or 
hours,  but  it  may  be  left  out  of  account  in  a  comparison.  It  required  the  best  efforts 
of  each  observer  to  attain  the  results. 

The  telescope  used  by  Dr.  DRAPER  is  an  ii-inch  photographic  refractor,  made  by 
ALVAN  CLARK  &  SONS. 

The  minimum  visibile  for  such  an  aperture  is  14.4  on  ARGELANDER'S  scale. 

In  the  accompanying  Table  A,  I  have  given  a  list  of  the  stars  laid  down  by  BOND 
which  are  found  upon  two  photographic  prints  which  Dr.  DRAPER  has  sent  me.  Table 
B  contains  a  list  of  such  of  BOND'S  stars  as  are  exterior  to  the  brightest  central  nebula, 
and  absent  from  these  two  prints.  Naturally  the  original  negative  would  show  far 
fainter  stars;  but  in  my  comparisons  I  have  been  confined  to  the  use  of  these  two 
prints,  and  I  have  included  no  star  in  Table  A  which  is  not  shown  in  its  true  position 
on  both  the  photographic  prints  examined. 

*  A  private  letter  of  Dr.  HIHHHXS  informs  n.e  that  in  a  photograph  ,,f  tin-  speetrnm  ,,f  tin-  nelmla  taken  in  April, 
1882,  five  Hues  are  shown ;  the  four  previously  known  in  the  v.s.l.le  speetrum.  and  a  n. 


228 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 


Taking  Table  A  we  see  that  stars  as  faint  as  11.5  are  plainly  and  well  shown. 
No.  793,  which  is  immersed  in  nebulosity  and  is  only  of  the  11.7  magnitude,  is  yet 
clearly  seen. 

There  are  shown  in  the  photolithographs  five  stars  fainter  than  the  13.0  magnitude, 
viz,  Nos.  435,  650,  653,  657,  778,  whose  magnitudes  on  ARGELANDER'S  scale  are  13.1, 
13.1,  139,  13:1,  13.1,  respectively. 

In  Table  B  the  brightest  star  is  of  the  11.9  magnitude,  and  this  star  (808)  is 
marked  variable  by  BOND.* 

TABLE  A. 


BOND'S 
number. 

ARGELANDER'S 
magnitude. 

Remarks. 

BOND'S 
number. 

ARGELANDER'S 
magnitude. 

Remarks. 

427 

10.7 

Very  faint  in  photograph. 

670 

10.8 

- 

430 

11.7 

Very  faint  in  photograph. 

685 

8-3 

435 

I3-I 

Very  faint  in  photograph. 

690 

10.3 

•449 

10.5 

700 

ii.  5 

453 

II.  2 

Very  faint  in  photograph. 

705 

ii.  5 

467 

8.7 

707 

II.  2 

479 

10.  0 

708 

9.6 

505 

'       9-6 

709* 

12.3 

506 

II-  3 

724 

10.5 

523 

10.  I 

732 

ii.  5 

551* 

10.  I 

734 

9.0 

554 

9.0 

741 

IO.O 

558 

10.7 

746 

10.8 

570 

9.4 

750 

10.8 

580 

12.3 

757 

IO.O 

598? 

12.3 

Doubtful  in  photograph. 

778* 

13.1 

635 

10.5 

781 

10.8 

647* 

12.  I 

Just  visible. 

784 

10.8 

650 

I3-I 

785                       10.8 

653 

13-9 

793                       ii  -7 

In  the  nebula  ;  faint  in  pho- 

657* 

13-1 

822 

10.7 

tograph. 

663* 

ii.  7 

848 

9-9 

669 

9.8 

863*? 

12.5 

Doubtful  in  photograph. 

TABLE  B. 


BOND'S 
number. 

ARGELANDER'S 
magnitude. 

Remarks. 

BOND'S 
number. 

ARGELANDER'S 
magnitude. 

Remarks. 

508 

12.3 

641* 

14.8 

5>o 

I3-I 

652* 

13-9 

5i6 

13-5 

767 

13.9 

524* 

12.5 

789 

14.8 

532 

14.2 

797* 

15-0 

545 

I3-I 

805 

13-9 

552 

14.9 

808* 

II.  9 

566 

13-3 

826 

14.8 

615 

14.2 

832 

13.9 

N.  B. — Those  stars  marked  with  the  asterisk  (*)  were  reported  to  bo^variable  by  BOND. 
*  It  should  be  borne  in  mind  that  if  these  minute  stars  have  a  slightly  reddish  tint  that  also  might  account  for 
their  absence  in  the  photograph.— [H.  D.] 


MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION.  229 

The  stars  marked  with  asterisks  were  counted  variable  by  BOND,  and  the  posi- 
tions of  some  of  them  in  the  lists  are  suggestive.  Undoubtedly  No.  778  was  near  its 
maximum,  and  this  instance  suggests  an  important  application  of  photography  in  the 
detection  of  variable  stars. 

It  is,  however,  when  we  examine  the  details  of  the  nebulous  structure,  as  shown 
by  the  photograph,  that  we  can  best  appreciate  the  astonishing  advances  which  have 
been  made. 

The  most  important  evidence  to  be  obtained  from  Dr.  DRAPER'S  photograph  is  in 
relation  to  the  brightness  of  the  different  portions  of  the  Huyghenian  region.  The 
photograph  gives,  of  course,  the  brightness  as  shown  by  the  chemical  decomposition 
of  the  salts  of  silver  on  the  plates  used.  This  is  not  directly  comparable  with  their 
brightness  estimated  by  the  eye  or  measured  with  a  photometer.  Still  it  must  be 
remembered  that  the  plates  used  by  Dr.  DRAPER  are  sensitive  to  rays  lower  in  the 
spectrum  than  6,  for  example,  and  therefore  represent  the  results  of  eye  observations 
far  more  nearly  than  ordinary  sensitive  (wet)  plates  would  do.  The  brightest  part  of 
the  photograph  is  in  the  region  about  A,  agreeing  with  all  my  own  photometric  work. 
D  is  faint;  about  as  bright  as  E.  My  photometric  results  of  1880  give  in  the  mean 
E  m  0.95  D.  The  earlier  ones  make  E  relatively  considerably  fainter.  My  conclu- 
sion from  my  own  observations  was  that  E  is  now  brighter  with  respect  to  D  than  in 
1878,  and  from  the  whole  series  pf  eye  observations  of  all  astronomers,  that  E  has 
certainly  increased  in  brightness.  Formerly  it  was  undoubtedly  fainter  than  D.  This 
was  so  from  1850  up  to  1878  and  even  1879.  Now,  by  my  photometer  observations, 
and  by  the  testimony  of  Dr.  DRAPER'S  photograph,  it  is  about  of  the  same  brilliancy. 

In  the  case  of  the  mass  A,  also,  the  photograph  gives  undeniable  evidence  of 
change.  For  quite  a  time  A  was  not  the  brightest  mass  of  the  nebula ;  .now,  it  cer- 
tainly is.  In  the  photograph  F  is  the  brightest  of  the  three  masses  F,  G,  H.  My 
photometric  observations  agree  with  this.  The  mass  N  is  the  only  one  whose  relative 
brilliancy  is  materially  different  in  the  photograph  from  its  brilliancy  as  given  in  my 
own  results.  I  attribute  this  in  part,  at  least,  to  the  effect  of  the  proximity  of  the 
image  of  the  star  685  to  the  mass  N.  This  would  alter  the  relative  chemical  effect  of 
the  masses  near  it  and  those  far  from  any  such  influence,  to  some  degree. 

It  will  be  of  interest  to  compare  Dr.  DRAPER'S  photograph  and  Fig.  7,  which  gives 
a  drawing  made  by  myself  through  tourmaline  plates,  arranged  so  as  to  cut  off  the 
fainter  portions  of  the  Huyghenian  region. 

The  shape  of  the  dark  space  bounded  by  E,  F,  and  Gr,  and  I,  is  the  same  in  both 
The  Sinus  Lamontii  is  alike  in  both ;  the  darker  space,  S,  connecting  T  with  the  iS' 
Lamontii,  is  also  similar,  and  so  in  other  cases.     The  photograph,  however,  represents 
relatively  far  more  light  than  Fig.  7,  and  if  we  were  arranging  our  work  in  a  seri,  > 
in  the  order  of  light,  we  should  put  first  BOND'S  drawing  (the  frontispiece),  then  DRA- 
PER'S photograph  (Fiff.  40),  and  lastly,  Fig.  7.     To  comprehend  the  extreme  faintnesi 
of  some  of  the  nebulous  masses  represented  in  the  photograph  a  study  should 
made  of  the  Messierian  branch,  of  the  regio  Picar<1iana,  and  especially  of  the  mass 

of  the  Index  Chart. 

The  comparison  of  the  photograph  with  the  drawing  of  Lord  Roi 


230  MONOGRAPH  OF  THE  CENTRAL  PARTS  OF  THE  NEBULA  OF  ORION. 

and  of  BOND  (frontispiece),  or  with  the  Index-Map,  should  be  made  in  order  to  appre- 
ciate their  marked  agreement. 

I  have  not  given  such  comparisons  in  detail  for  the  obvious  reason  that  this  pho- 
tograph comes  as  the  beginning  of  a  new  epoch  in  such  observations,  and  it  will 
receive  its  proper  discussion  as  the  very  first  of  a  series  of  exact  and  automatic  repre- 
sentations which  we  must  thank  the  skill  of  Dr.  DRAPER  for  inaugurating.  I  feel  that 
the  present  memoir  receives  a  new  value  in  that  it  brings  the  work  of  the  old  period 
together  for  discussion,  and  leaves  a  clear  field  for  the  employment  of  the  new,  and 
far  more  satisfying  methods.  At  the  same  time  I  must  point  out  that  the  evidence  to 
be  derived  from  this  photograph  lends  a  great  strength  to  the  best  previous  drawings 
of  the^  nebula,  such  as  those  of  Lord  ROSSE,  BOND,  and  LASSELL.  It  has  always  been 
easy  to  object  to  drawings,  estimates,  and  even  photometric  measures  made  on  objects 
of  so  great  faintness  and  difficulty  as  the  nebulae;  and  the  conclusions  derived  from 
such  work  have  often  been  met  with  the  criticism,  easy  to  make  and  hard  to  answer 
that  the  personality  of  the  observer  was  so  much  to  be  feared  that  such  conclusions 
remained  doubtful.  I  desire  to  emphasize  the  fact  that  all  the  important  conclusions 
as  to  the  present  state  of  the  nebula  which  I  have  derived  from  an  examination  of 
such  drawings  as  those  I  have  mentioned,  are  confirmed  by  this  photographic  repre- 
sentation, at  least  in  so  far  as  it  is  capable  of  giving  any  evidence  at  all,  and  that  it 
comes  to  confirm,  and  not  to  destroy,  our  confidence  in  the  faithful  work  of  competent 
observers  by  the  ordinary  methods. 

(  U.  S.  NAVAL  OBSERVATORY,  Washington,  D.  C.,  December,  1880.  ^ 
(  WASHBURN  OBSERVATORY,  Madison,  Wisconsin,  May,  1882.          $ 


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S  O  LV 


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